Hi!
Sheet metal screws work fine in plastic mounts(Craft-Hays). Have been using them on many Q-500 models (424).
But another way of doing it is to drill and tap the mounts.
Drilling and using nuts on the opposite side isn't suitable on many models...and those nuts are also hard to reach when you have to tighten the screws.
Using Ca glue to secure the engine to the mounts when drilling is one way of doing it.
But most of the time I just hold the engine firmly to the mount and drill just a shallow hole with my electric hand drill, then take the mount and using my bench drill, drill through the mount. Then I tap the hole (or screw in a metal screw if its a plastic mount)and use an Allen head screw to secure the mount to the engine.

Then I take a long drill and using my electric hand drill I drill shallow holes having the engine lugs as a guide.
I then remove the one bolt and put the mount in the bench drill, securing it in a wise, and drill the remaining 3 holes.