You obviously have the skills to do the mechanical work, so fitting the encoders should not present too much trouble.
First remove the resolvers, then once you have the encoders in hand you can get a better idea of what it entails, as long as the physical size of the encoder body will enable any back cover to go on and it is just a question of working out the mounting method and come up with a suitable helical coupling.
If Barry has encoders, then you can get the dimensions etc from him.
Al.