[Brian]

No need to pull the crank to fit the tube, only to fit Reds conversion. Generally, these old 21 studs will leak more as the rear main bearing [and others], but mainly the rear main [no pun intended], wears.
If you just want to freshen it up without spending lots of money, you can replace the rear main bearing with crank in place, but fit that tube at same time and you'll really notice a change in leakage. These are cool old motors and you'll be amazed at their longevity. And I like your woodie. Obviously, do a clutch job at same time.