Randy Bimrose did awesome work on a "dead" Diana model 60 for me a few years ago. He tends to get booked up way in advance though.
Just curious...does Mr. Slade refuse to USE JM parts, or does he just not BUY them? I'm wondering if you could buy the parts yourself and send them to him with the rifle.
You might ask Pilkington airguns, if they would work on these. They primarily deal in current state-of-the-art match hardware of course, but they do rebuild FWB 300's and such.