Campgems...That's what I call a real "Yard Sale"!

Here is a photo of sheetmetal screws holding a .61 in a Tiger 60. The screw length should have been such that three threads are showing after final assembly and tightening. Note the clearance between the engine mount and crankcase. No problems to date.

Drilling and tapping works well also.

"Through Bolting" is the most secure method.

I use a small flat washer between the head of any chosen fastener, and the engine mounting lug.

No matter which type of fastener you choose, I recommend after a few flights, to check the fasteners for tightness, even though this might require removal of the spinner/prop/muffler/cowl, etc.

Furthermore, every now and then before flying (and before starting the engine) I think it's good practice to grab hold of the engine and give it a test for mounting tightness/security. You might get surprised one day[8D]