ORIGINAL: RPM1
4-40's will work for up to a 40 size, use 6-32 for mounting a 40 to 90. as for where to get bits... Home Depot, Lowes, Ace Hardware any other place like that should have them. The should also have tap and die sets and the like. Make sure you use T nuts on the back end. Also make sure you use locktite on the screws.
Probably a dumb question here but, why tap and thread these holes if you're just going to loctite them anyway? I was under the impression that, if you don't plan to remove the engine from the mount, self tapping screws are the way to go. On the other hand if eventually want to remove the engine, the tap and thread method is the way to go.