The best tool for modifying the engine mount might be an ordinary file. It is not fast but it is easy to control. A sanding drum mounted in the dremel, or drill, would probably work. It will heat the metal, which may or may not cause problems.

I'd probably try putting a switch in the current tool; or maybe it's still under warranty. If buying a new cordless tool I'd go for the 10.8 volt variable speed kit. $70 with free shipping.
http://www.amazon.com/Dremel-8000-02...4959253&sr=1-2

For a corded tool:
This set is $120 (with free shipping) and would be my first choice
http://www.amazon.com/Dremel-400-6-9...4958860&sr=1-1
Second would be the $84 (with free shipping) set:
http://www.amazon.com/Dremel-3956-02...4958860&sr=1-3
Both are variable speed and have the flex shaft, which is really nice to have.

I've ordered a couple of tools from Amazon. They're not always cheapest, but often they are. Many items have free shipping and delivery time on my orders was just a few days.