There are two solutions to this. If your engine is made where the screw holes go all of the way through the engine then you could easily just buy a longer set of screws and locknuts (the ones with the nylon insert in them).

However if that won't work then the next best thing is to use lock washers on the screws. There are a few different types so you can try each one out and see which works best for you. Your local hardware store should stock them. They may also stock the screws/locknuts that I referred to above. If not they may be able to order them for you. Just make sure you bring your original screws with you so they can be matched up.