I'm assuming you have the 1st generation Miss Bud with the Solid drive shaft not the flex shaft, those were the ones with servos glued to the bottom. There is no easy way to replace those servos and you will probably be doing some repair work before your done.
A Heat Gun or real good hair drier will help soften the glue some, then gently pull or pry the top plate of the servo bracket. off then the servos themselves. I replaced the wood with an aluminum brkt. and mount.
Heres one installed.
this is another version, in a Formula
An .18 engine would be a much easier and a better suted upgrade for that boat than a .21.