Hey ya'alls.....I can vouch highly for the Zen50/OS70 surpassII combo. I can't say enough about huh Mav , really it is a great combo for this plane. The power of the 70 is really good, I wish I had a Saito 72 to compare with. I just don't see this plane needing wanting anymore then the OS though(not that Saito is more, don't know the answer to that).

Totally unchallenged vertical, excellent power band for pattern flying. Very good breaking on downlines.
It fits totally within cowl, the only cut I made was for just the top of the valve cover(slighly visible on bottom and looks custom(mounted inverted)).

I ran a remote glow(dubro) to the side of plane directly opposite radio switch. I used stock exhaust, just cut a very small exit for it, hardly noticable. Some guys like the tuned mufflers that use the center cavity in the belly pan. I opted for the stock exhaust I already had, and save the $150+ the pipe set up ran. I wanted to get plane done and PRACTICE. That exhaust set-up wouldn't make me any better, practice does.

I've said before, if you want speed go with a 2 stroke of some sort. Matter of fact I had a TaiJai with a 2stroke 46/and pipe that was a bit faster, but COULD NOT hold a candle in uplines or downlines to the OS.
I'm not sure how a 61 2stroke would perform. Unquestionably you would have the fastest Zen on the planet. If that's what your wanting it would work. If that would be usable/controllable power is a real question. Fast and powerful don't play big roles in precision. Usable is what you might want to look at.
For me the 70 keeps it very controllable with plenty of power to spare. It's a really good practice plane that I'm using to enhance precise flying.
Be conservative with your control throws for first flights until you get to know it. It will cut the air like a knife, and turn before you can think. First flights were interesting to say the least, I've since tamed low rates down drastically, mid rates are mild, high rates you literally can't count rolls.

Hope this helps, your in for a treat with this plane, it's a well kept secret. ssshhhh..... C-ya