You really should use a new mount if you can. Drilling new holes does weaken the mount some. Now with that said, I have reused a mount before. Here's what I did. Before you drill new holes in the mount for the new engine fill in the old holes. I screwed bolts into the old holes and then cut them off flush with the mount. This will help strengthen the mount some, but it's still going to be weaker than a new mount will be.

If you do reuse the mount please consider it temporary until you can get a new mount. Trust me, having a motor mount break in the air isn't good at all.

Hope this helps

Ken