Air Leak -crank seal repair

Any vintage bike mech's there? My bike is leaking air bad. Lean-out is causing it to rev on it's own after starting. It can run at appox 4-5k rpm's steadily if I almost completely close-off the mouth of the carb with my hand! Is it possible to replace the crank seals without splitting the case on a '75 yamaha dt125? For left side, I'm hoping to pull the armature and take a look to see if this is possible. I think I've pretty much ruled-out all areas of leakage other than crank seals or main case seal. Occationally, if the bike misfires, I see exhaust leaking from the left side cover. This can be observed with the point plate cover off.
Any help would be greatly appreciated. Thanks, -Jeff

Good news, thanks for the reply.

Is there a special puller for the armature? Is it pressed onto the crankshaft?

I got it off and the left crank seal replaced. That was definately the problem. The seal was trashed. The bike runs good now. Thanks.