As others have pointed out--the felt seals can shrink. When my '69 144S sat for some months and then went back on the road it developed a rear seal leak. I found the felt seal had actually split. You can replace the seal housing with one from a 164 as has been mentioned. A housing from a B21, B23,B230 will work too -- but remember to use the two pan bolts from the later motor as they are metric threaded.
As for the motor stopping -- could be many things -- one possibility is the coil winding in the Bosch electronic distributor could be shorted. I had to replace mine not so long ago. In my case it behaved in typical failing Bosch distributor manner--it would cause the motor to conk out went hot--restart when cooled off--until final failure. -- Dave