A good engine for a 70" bipe would be an SK 90 from www.kangkeusa.com. Turns a 15-6 prop very nicely. $100. The Tower .75 is also excellent, but tends to like small props. $90.

I have 2 of each. I run my SK 90 on a 15-6 and it pulls strong. The Tower runs on an 11-8 or a 12-6 and is speedy.

There is 1 ST 90 at my field and after 4 years and as many owners, it is finally broken in and running well on a 13-6. I would go with the SK 90 and the big prop.

Another thing about the SK is the carb is excellent. It 1-flip starts, as does the Tower, and it idles all day without quitting. Great mid range, too. I have many OS engines and the SK carb is every bit as good. The SK does take a good while to break in as do many large, ringed engines.