If you like OS you choose between (Dont know about the 61fx):

Twostroke:
46ax
50sx

Fourstroke
52fs
70fs


Yes I runt the .70fs and it's got more power than I need. Plane takes off almost immediately and can continue straight vertical.
The .70 is absolutely all you need for any type of flying IMO, but I guess a smaller and cheaper Twostroke will also do the job.
Make a search on "SSE" or "Somethin'" here on the forum and you will find hundreds (if not thousands) of posts concerning everything about the SSE.
When you read some of it you will get an idea what engines are mainly used on this plane and what performance they give.

As I said, the good part of using one of the mentioned twostrokes is that you can run a 11 or 12 inch prop. This mean that you can use the stock landing gear.
Althou my .70 can be run on a 12x8 it gives to much speed and a 13" prop is desired = need a new, higher landing gear.

If I were you, choosing from OS and mainly twostrokes, I'd go for the 50sx or 46ax.
One of tese engines will give good power, fit in the stock engine mount and need no landing gear modification... Plug and Play