I just pulled apart my first of the two bolt backplate engines and definetely like the 4 bolt engines better... they convert cleaner... I haven't trimmed the engines down or anything just took them out of the housing.

and to answer a question I've heard before, the jug won't bolt onto the 4 bolt backplate engines, the bolt spacing is different by a few thousandths, I might try modifying the jug on one I have that's kinda screwed if I can free it up, I don't know if the connecting rod bearings are the same size