All this is a compilation from information I have gleaned from: Toyota 4 Cyl. Engine Information, the 20R Head on a 22R Block thread and other places. Please don't hesitate to contact me if you have any questions, comments, or suggestions. I have just sorted through lots of pages and E-mails researching for my own 20R conversion, and have now compiled all of this with my own experiences, hopefully for the benefit of others.
A little-known secret amongst 22R builders is adjustment of the camshaft (not ignition) timing. For better overall performance, advancing the cam timing approx. 3 degrees seems to be the magic number. An adjustable timing gear can be used to increase performance as well as bring the cam timing back into specifications due to the centerline changes. Any camshaft from any 2xR series engine will work. Camshaft design did not change from '75 (20R) until about '85(22Rx). However, for maximum performance, switch to an aftermarket cam, such as a LC Engineering Street Performer.
After the 20R head conversion is completed a jump from regular to plus, or even premium will be required to prevent pinging. I currently have to run premium to prevent pinging. As soon as I have some more free time I will mess with the cam timing to try and change this.
Several modifications to the air/fuel induction system will need to be made. The exact modifications depends your budget, performance expectations and other factors. The easiest modification is to merely use a 22R carb to 20R intake manifold adapter. The method I ended up taking was purchasing a kit that came with an Offenhouser 20R intake manifold, Webber 32/36 carb, and a 11" K&N airfilter. This has several advantages over the stock 22R carb, mainly increased performance, and decreased complexity. No mater what path you choose, you will need to modify the 22R throttle linkage. A $11 adapter bracket that makes the job much easier is available from LC Engineering. You still have to cut the stock linkage appart at the seam, but do not have to fabricate the bracket yourself.
A name that keeps coming up again and again is L.C. ENGINEERING. They are a bit expensive, but top of the line and all they do is hot-rod Toy motors.
I was driving my truck home from college for the summer ('98) when about 40 minutes from home I heart this "Thwack" sound from kind of near the engine. The truck was still appeared to be fine, so I continued on, until about 4 miles from home. There is a large hill on the freeway about 2 miles from my exit so I went to downshift to 4th gear. It came out of 5th but never went back into gear. I flicked on my hazards and managed to sneak it into 2nd finally, and limp home. The next morning I went out and inspected the truck to find out what had happened. I found that the throwout fork was knocked off and pulled a mangled bolt out that was jammed in the boot. What had happened is an mechanic at Bud's Repair in Klamath Falls had done a poor job rebuilding my transmission a month beforehand. He didn't tighten down one of the bellhousing bolts, and it came loose and knocked everything loose. I called them, and at one point I mentioned that I was thinking about rebuilding the engine (for more power, and due to a previously blown and repaired head gasket). They offered to find and do the machine work on a 20R head, and replace the clutch, so I agreed. I put the truck into Low 1, started it in gear, and drove it into our shop, where it ended up sitting for the next 6 months.
It ended up taking me all summer and threats of legal action to get my heads (sent the 22R down to have the valves installed in the 20R head) back from Basin Machine Shop (part of Bud's Repair). I found that they had done a very poor job of surfacing the head, and hadn't properly decked it. I then sent both heads off to LC Engineering, the 20R for machine work, and the 22R as a core, for payment on the machine work. LC promptly got my 20R head back to me, which was by then 4 days until I was to return to college. The rest of the engine was completed and ready to go. With LC's reputation for quality and my previous good dealings with them, I was sure I could get the head on in time. However when I opened the package water began to drip off the head, and upon further inspection I noticed corrosion in a couple of spots. Then, the worst of all, I realized that the camshaft retainer caps, valves, and other parts were missing. I was not happy. I had spent $150 to UPS the heads there fast, and had let them know the urgency. I packaged the head backup and sent it back, telling them I was extremely angry, and that they better return the head to me intact, and cleaned. I returned home several weeks later to get some more work done, including the cleaned and intact head. All was well, until I tightened down the camshaft. It would not rotate at all, and further inspection revealed that the camshaft retainer caps were offset by almost 1mm. I was forced to return back to college before I could contact LC again. When I was able to get a hold of them they told me exactly how it could not possible be their fault, but would fix the problem if I would return the head to them (sounds pretty fishy to me too). I got a big favor from my brother, and had him pull the head (with intake manifold and carb attached, for his convenience) and send it to LC. Christmas finally came, and I was able to return home once again, this time to finally finish my truck. LC had done a poor job of packing the head for it's return and the aircleaner hold down stud was pretty bent. I was able to bend it back to working, but not original functionality. However everything else worked and I now have my truck running once again.