ORIGINAL: Mr67Stang
I would think, but may be wrong, that the addition of floats would push your CG aft so the larger engine may not hurt your CG as bad as you think.
Sorry, but floats shouldn't affect C-of-G at all. The re-balance with floats is accomplished by adding weights to the floats, rather than the airframe.
Depending on the engine, the battery may need to be moved aft in the Nexstar if a larger engine is used, but it is no big deal at all.