Christian, unfortunately the blocker ring is a difficult part to find. I sometimes see these transmissions selling for cheap on Facebook Marketplace, I have not had problems with sealing the solenoid. I just install the new seal as part of the rebuild. You may want to contact the Vintage Auto Garage. They sell parts, mainly the control stuff, but they also have some spare parts as well. I was fortunate that my transmission damage was limited to the synchro drum and every transmission gear had damage. I found the gears on Ebay over a six month period. The overdrive was in great shape and was just cleaned and inspected.
https://vintageautogarage.com/borg-w...saAqj_EALw_wcB
Best of luck.
Rick