Why are you not cleaning the original and either cleaning lubing the bearings or replacing. bearings shouldn't cost more than $10 a piece. Pop the old ones out and go to your local bearing supplier and have them sized and sourced. Such a simple job to do. Clean the commutator bars while you're in there.
Sealed bearings- You can always pop the seals out of a sealed bearing and wash the bearing in Varsol (paint thinners) and then lube the bearing up in EP2 grease or similar, then pop the seals back in. If the bearings are seized/rusted then replace. It's an intermittent motor, ie occasionally running with little to no load. Nothing fancy here.
It's a 40 year old motorcycle and you'll have to learn to start rebuilding parts in order to keep it alive. Half the joy of owning things like this is taking the credit for being able to "make parts" and having them work. This site is full of such endeavors and we give credit where credit is due..