Bounty,

Usually the allen screws that go through the engine mount are 3mm screws. Are you sure you didn't get a 3.5mm screw? (GST engine mount uses this size screw and a few other 1/8th scales do too).

If it is a 3mm screw, what I have done in the past, was I carefully enlarged the crankcase mounting holes with a drill press to 3mm to clear the screw properly. I had to do this to one of my piccos before.

Most engines have the same size clearance holes cut out for the engine mounts, but sometimes, some are a little tighter.