Michael,
I purchased my 701 with a 670 rotax (built by rotax rick) already in it.
Things I like about the 670
Great power to weight ratio (92hp @ 100lbs)
Simple engine, less to go wrong, small size leaves plenty of room to work on it.
Initial cost and rebuilding is much cheaper than aviation engines.

Things I dont like
2 cycles have reputation for stopping suddenly with little or no warning. So far (120hrs) this has not been my experience, but something I think about and try to keep in mind.
Recommended TBO is less than other aviation motors, although still cheaper in the long run.
Have to add 2 cycle oil, on longer trips, you need to have it along with you.
You have to watch EGT very close, easy to get lean when descending (this is something I have gotten used to, not as much a problem if you are careful. )
The weedeater sound....very LOUD weedeater sound.
Also higher fuel consumption than 4 cycle.

Overall I would recommend the 670 rotax for the 701,
I do think it requires being more careful in operation, but a good choice.

If I were to have my choice in currently available engines, I think I would probably prefer a Viking, nearly the same price and 4 cycle, much quieter, parts easier and cheaper to find, much better fuel economy and still good power to weight ratio.

Carl