ctc....
I don't know if "the Doc" is rebuilding them anymore, or if you can buy or borrow one of his tranny rebuilding CDs & do it yourself with all new & tight parts - as many new as possible.
Alternatively you could try to find a local expert shop who rebuilds 901 transmissions for 911, 912 & 914 - & have them do it for you if not comfortable with you doing it.
Also, the cases can get worn beyond what new bushings etc. can compensate for - so you may also need to find a better used or new/NOS case (when my 914 Tech rebuilt my 73 2L trans, they had to get a replacement 72 case due to excessive wear, when my rolling resto was done back in 80-83). IIRC they had an NOS case or two at AA a while back.
Additionally - I'm not sure which MY 914 that you're working on now, but if a 70-72 tail shifter - then you can either:
A. use the 72 MY tweaks & parts that tightened up that MY's tailshifter had done by the factory to improve its shifting (IIRC that's covered in the Factory Repair Manuals' 8 Binder set, if a post 1972 issue) - or -
B. convert your tailshifter with side-shifter parts (which was commonly done on 70-72 rebuilds back in the day), & just save the tailshifter parts for any future owner who may want to put it back to all original. (I look at that "upgrade", SSI heat exchangers, etc. as "in bounds" for originality, so long as done period correct.)
PS - You might also get more input in the Garage Forum for this question.
PSS - As an additional option C - you could get another sideshift or converted tail to side shift tranny to install for your driving, & just save & store your original tranny for the future (assuming it's the original, & assuming that rebuild parts may be more available in the future).
Also, since the 901 is shared with the 911/912 cars but flipped for tail-dragger use, also check in those parts source areas for those that you'll need which are interchangeable between both configurations.
Cheers!
Tom
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