One thing that I noticed about the ABC engine is that the inside of the head is CLEAN and has a 40 cast into the bottom. I seems to fit the liner perfectly???
Not to flog a dead horse, Frank, but just for future reference, the portion of the head that goes into the liner should be a very close slip fit, ideally to the point that it's hard to push into the liner if it gets cocked even slightly sideways - nearly the same as the piston fit. Under .001" clearance. Production engines don't have this, but the semi-custom engines like the RO-Jett and PA, the head/head button is fitted to each particular liner, and they are sometimes very difficult to get started straight enough to go in. I just tried to measure it, and while I can measure to a few ten-thousandths of an inch, a defunct RO-Jet 61 liner and head measure the same to the tenth. With both parts at the same temperature, I would guess its about .0002-3 interference fit, that is, a very light press fit.
I don't know the exact dimensions, but making some guesses, making a 46 out of a 40 by just increasing the bore will probably result in more than .020" difference in the diameter of the cylinder - which is so much that is this alone could probably cause your lost performance. The extra space reduces the combustion pressure but is otherwise useless for burning fuel, it's much worse than just grinding out the combustion chamber or adding head gaskets, since more-or-less no fuel/air mix ever gets there, it's just dead space.
What is very likely is that someone had some engine parts lying around, the holes lined up, so they just put the parts together to make nominally complete engines, to sell them - a Frankenmotor. Don't blame the guy you bought it from, he may have gotten them that way, too. But I *have* seen people do things like put "40" heads on "46" engines on purpose, to "reduce the compression" which has inexplicably become Step 1 in the 12-step plan to "make it a real stunt motor", i.e. screw up an otherwise perfectly good engine and make them run like crap. Mission Accomplished!
That's why they tend to get passed around from person-to-person-to-person, it doesn't work, they sell it, it still doesn't work, the next guy sells it, etc. There's a Fox 45 "Compact" around here that I have seen 4-5 different times, different people have it every time, it still has the same issues, and has gotten sold multiple times. I haven't seen it in about 10 years, maybe someone finally put it out of its misery.
I am sure Randy has all the parts and compete engines, if you want one. As noted, it would be possible to machine a new head, or to sleeve the existing head, to make it fit. But, given that one of them is worn out, the other also needs a new liner, and there are many other, better, stunt engines widely available, you have to question whether fixing it makes sense.
Brett