I'm with Ed--nylon mount, socket head sheet metal screws, eased in with a drop of oil. Engines do vibrate for sure, but if I torque down a SHSM screw with an L-wrench finger-tight, they stay tight indefinitely. Use the largest size screw that will slip through the engine mount lugs--#6 for smaller engines, #8 for medium size, #10 for larger sizes. Pilot hole in the nylon mount should be just a bit smaller in diameter than the OD of the screw threads, then you will not have screw heads breaking off... 1/8" bit for #6 size screw. Steel threads in nylon mounts hold very well, no vibration problem, no stripping out. Use a length of screw that reaches all the way through the thickness of the beam.

Nylon locknuts on machine screws hold well, but you need access to both ends to mount or remove the engine, whereas SHSM screws all you need is access to the screwhead.

On occasions where I've had to use aluminum mounts, I like to drill & tap the beams, then fasten with socket head machine screws and split lockwashers. Normally I use an aluminum mount only where I need extra noseweight. It does take some care to position the mounting holes precisely. If you screw up, you can often salvage the situation by drilling out the tapped holes and going to machine screws with nylon locknuts on the other end as a last resort. Measure carefully, mark precisely, punch the mark with a steel punch to keep the drill bit on center, steady the mount with a drill-press vise, and definitely use a drill press rather than hand drill. (Nylon mount is so much simpler. And lighter. And cheaper.)