That's been a concern of mine but so far the lenses I've bought don't seem to be bad. I trust Guy would have let me know if I had a real bad one.
............. I guess the less good elements got reassembled and ended up on EBay
Given the price for conversions I'm some surprised that no-one ever came up with a custom follow focus rig designed to take some of the pain out of focusing the original internally focused design. Something like a geared down, servo-controlled motor with an extended focus scale driving the gear on the lens. Considering what Van Diemen and Century conversions cost that could have been pretty cost effective.