Dubro carries 3,5 millimeter screws...
Dave, the ironic thing is that M3,5 screws are VERY hard to find over here too... decent ones, that is... the cheap stuff is all over the place.
One time I actually bought M3,5 allen head screws with an imperial head size (and a set of matching imperial hex keys) because I couldn't find anything else...
Oh, and about using alloy screws... that may be good enough for keeping your backplate on, but since you are mounting your engine with these screws, I'd look for something more solid...