There is significantly more to IFR approval than just the engines issue.
For starters, you need to have a trim system of some sort for the cyclic in order to satisfy the requirement for self-centering.
There are a whole host of single-point failures on a single engine helicopter that needs serious thought before we start to advocate it whole heartedly.
Personally, I wouldn't think of going IFR without a second attitude indicator - a turn and slip needle isn't really good enough (even though that's permitted under FAA regulations).
A autopilot system, not just a stabilization system should be necessary, if not mandatory for single pilot- (and how do you cope if it fails, one might ask???)