In a plastic engine mount, socket head sheetmetal screws work better than machine screws. All you need to do is drill pilot holes. It really helps to have a numbered drill index. Harbor Freight sells a nice set for about $10.
In an aluminum mount, it really helps to use high quality taps...not the junk sold by the hobby industry. Go through an industrial supplier for at least tungsten carbide.
Use Rapid Tap fluid or equivalent.
I chuck the taps up hand tight in a slow speed drill motor and make short work of it. The key is having the best taps money can buy. In the long run they are money well spent.