My opinion would be that since you already have the .61FX you might as well use it. Buddy of mine built a .60 size Sportster back in the late 1980's and used a Webra Speed .61 and that was a real nice flying airplane.
Try and keep it as light as possable...the .61FX will be a good combo...not spectacular, but nice.
If you're in the mood to buy another engine, a .75 or even a .91 would make it closer to "spectacular"
The .61 size engines don't seem to get used much anymore...since you can get .75's and even .91's in the same size case (and wt. )
I have a .61FX, and it's a decent engine IMO. Mine will turn an APC 13x6 at 11,500 r.p.m. on 15% nitro Omega and a Slim Line Pitts style muffler.