To address a couple of issues you raised:
- I have no doubt that someone will adapt larger four cycle engines, but here's the problem- the Honda GXH50 runs up to 7800 rpm. The larger engines run at an rpm about 1/2 that. What's the big deal? The same reason 2 strokes make more power (though not twice the power) than 4 strokes for a given displacement- more power strokes in a given amount of time.
The popular Honda GXH50 (49.4cc) is rated at a maximum of 2.1hp @ 7000 rpm (yes, I know it is advertised at 2.5, but 2.1hp is from Honda)
http://www.honda-engines.com/engines/gxh50.htm
The next larger size Honda, the GX100 (98cc) is nearly twice the displacement, but is rated 2.8hp @ 3600 rpm.
http://www.honda-engines.com/engines/gx100.htm
The next larger size Honda than that is the GX120 (118 ccs) is rated at 3.5hp @ 3600 rpm.
http://www.honda-engines.com/engines/gx120.htm
Add to this that the GXH50 weighs 12.1 lbs., the GX100 weighs 23.4 lbs. and the GX120 weighs 28.7 lbs.
Therefore, the GXH50 produces .17 horsepower for every pound, the GX100 puts out .12 horsepower for every pound, and the GX120 puts out .12 horsepower for every pound of weight. The GXH50 produces 140% more horsepower per pound than the other two. The GXH50 weighs only 51% what the GX100 weighs and makes 75% of the power. Noce advantage.
This does not even take into account the size of the engines.
I say all that to say this- you would have to go way up in engine size to realize any performance benefit- and then the size and weight of the engine would get impractical. Would there be more torque? Probably, and there may be quicker initial acceleration, but any advantage to the increased displacement is outweighed quickly by weight, size and inefficiency.
I only compared Honda engines, just because I could not very well compare all engines. I doubt that there are any huge differences from brand to brand.
- Next, in most states, an engine under 50 cc (and a couple of other requirements) results in the classification as a moped and there is no registration or insurance requirements- it does in my state (Indiana). Over 50 cc, it is a motorcycle with all those requirements. A 2 stroke by nature, is not as large and even an "overized" engine would not necessarily be recognized as overlarge. A four stroke, even 50cc, looks too big to begin with, even if it is legal. I feel confident in being able to explain myself if need be, to a judge if necessary...I happen to be a lawyer.
- I have seen kits out there with widened arms for 3 piece cranks. I cannot speak to quality, but I know they are out there.
