If the venerable 350 twin: I've changed them before by removing the cam bearing itself (there's 4 screws holding it on). It just takes removing the points plate (punch mark it first?) and the spark advancer, then the rocker shaft nuts/washers, then those 4 screws (and you'll likely need a new gasket, easy enough to make one, though) and tap/wiggle it off after breaking the gasket's seal. Then the shaft seal just pushes out.
I also fixed one up where the owner tried using a crochet hook to just pull the old seal out. That one needed another camshaft because the shaft was so buggered it ate the new seal he then installed. I don't think those surfaces on the camshaft are very hard?
Before you reinstall the spark advancer...note if the points are using 2 widely different gaps, like 0.012" on one and 0.016" on the other. If so, the points cam on the spark advancer is wobbling because the 2-3 plastic shim washers that used to be behind the points cam melted (around 1980 or so) and fell out. If you re-shim the points cam to 0.002"-0.006" max clearance between the hold-on washer and the rotating face of the cam on the advancer (when the advancer is bolted to the shaft) then the "L" spark will improve by about 300% and the bike will start first kick most all the time, again.
I use thin brass or aluminum sheet for these shims. They don't have to even be round, so long as they fit over the little mounting shaft. I punch out a center hole, then cut them with scissors to fit. Use at least 2 of them: using just one will make it crumble up. There were always 2 or 3 plastic ones when new.
What is happening to the advancer is: when the points cam is wobbly and not shimmed to stay parallel to that inner shaft, the cam starts to open the "L" points, then the slack collapses under the cam and the points close again. Momentarily later, the clearance is gone and the points get opened again. This causes the coils to start to discharge, then stop, then the field starts rebuilding, then points open again, letting the half-charged magnetic field collapse in a weaker spark. This doesn't happen at the R points because of the geometry of the arrangement.
I first ran across this decades ago when installing the Transistor Ignition on these bikes (the 450 does it, too). The left spark would be even weaker because the transistors hasten the field collapse by more completely shutting off, then turning more cleanly back on. The L coil would run hot at idle speeds and the Oscope traces would clearly show 2 triggering events coming from the points when running, with weakened "L" spark levels. Installing shims to reduce the clearance behind that washer was the key, and then the bike ran like new with no other work done.
