Looked at a bunch of them for by Bridgeport.
Ultimately, you have a choice of engineering your own retrofit (not for the uninformed/uneducated) or buying a kit and bolting it on.
I'd be inclinded to buy the pre-engineered kit which may or may not interface with your old servo's - check closely before you commit.
Mach has charm BUT it can't interface EASILY with servo feedback (long complicated story - in short, I wouldn't bother but surely someone with take exception to this).
Looked at Anilam but the cost got prohibitive due to downturn in business (proprietary system that supposedly uses a DOS backplane - I"m not a fan of ANYTHING Windows based for CNC).
Cost/benefit wise, I became fond of the AjaxCNC concept. Ajax is the DIY version of Centroid. Linux based and quite affordable at a reasonable price. Self tuning and seems to be reasonably "plug and play" if you use their "kit". They do a BOSS retofit which may be close enough to P&P (paraphrase and plagiarize).
FYI: the former Bridgeport engineers who helped design/build the early Bridgeport CNC stuff (IE: ExTrak, etc) now run their own CNC service/integration business. Interestingly, they sell, distribute and service Centroid. Says a lot about the product if you ask me....
You should/must also do a google for "cnc retrofit" and go deep into the pages that will turn up.