Parts for any of the pre-Lambda models are very thin on the ground and Stax shows no stock. Your best bet would be with any of the larger Stax distributors which have been in buisness since the 80's (Germany being very big on the SR-5's back in the day) or watching ebay.
As for running the bias a bit hot, the driver can in theory easily handle 500V bias but Stax have always been rather conservative with bias voltage in context of the D/S gap.
I just replace everything but then again I'm not normal. That said, the caps in my old SRM-T2 measured like new but replacing them with fresh units cured the offset issues I was having and raised the voltage rails up to spec again.
If I remember correctly the old SRA units were all AC-coupled so you might want to look into new output caps as well.
My board haven't arrived yet to no testing as of yet but people were experiencing odd things like the volume of the system drifting as if the bias supply wasn't working correctly. There were also some issues with the first supplies (as in they never worked) but I converted mine to the voltage doubler with a voltage divider at the end setup from the start so I never had any issues.
As for the exstata, I've been cleaning up the layout of the new boards to try and make them smaller for the last couple of days and I've gotten them down to 195x122mm which isn't bad for something which has the same number of heatsinks as the KGSS two channel board. A simple PSU coming up too and then I'll order some boards.