Hi,
Firstly, let me tell you that nothing 1/2 decent is going to be cheap. Secondly, the likelihood of finding something to fit straight in to your existing mirror cell and with the right focal length, will be about as easy as climbing Mount Everest. I would think it would be a far easier, cheaper and better option to sell that scope and buy one of the cheap Chinese APO's, which perform very well for what they are. With a 4"/F6 refractor its focal length is way too short to excel as an achromat, for higher power work, so you are limited there for starters. The Chinese APos are 4"/F8's and work quite well considering their price.
Contact D & G optical. Their smallest size is 5" but they also do custom work. They only do achromats but as good as an achromat gets. Lovely lenses plenty of $$$$
http://www.dgoptical.com/objective.htm
Bob Royce makes custom replacement refractor objectives.
http://www.rfroyce.com/restoration%20work.htm
Slightly cheaper would be Antares Optical in Canada
http://www.antaresoptical.com/atm.htm
Cheers,
John B