The Saito 56 will fly your Nexstar just fine. Probably a shorter take off roll but less top speed. I did a similar conversion 15 years ago and I really liked the way the S-56 flew my plane. If your mount will not allow the S-56 to fit then try to find a mount with the same bolt pattern but if you have to redrill and change the blind nuts thats not a big deal either.
Fred