I just bought an ernst adjustable engine mount for a .40 engine, and the bag comes with four screws to mount the engine.
Question is: are these reliable, or should I use bolts with blind nuts or lock nuts. Forgot to mention the mount is made of nylon

I find it easiest to tap it with a 4-40 tap, and put some machine screws in. As for the screws that came with it, they are perfectly reliable. It is only a matter of personal choice.