I have a .91 Surpass in my Sportster. They make a great combination.
Your .45 is prbably too small, and definately too light. The Sportsters build tail heavy, so you want spme weight up front. It would be great in the 40 size.
The .70 Surpass is a wonderful power plant...........for the 40 sized Sportster. Again, I thik it's too small for the 60 sized version. I had a 70 Surpass in my 40 Sportster. It was the best flying of all my Sportsters.
If your .61s are dependable, they'll be fine. Be prepared to add nose weight.
The FS 90 would be my choice. Don't follow the instructions where they tell you to move the firewall rearward. Build it as is, and lengthen the nose. You may get by without nose weight, if you do.