Installing new left side engine case. Need to stamp it with the old case serial number. Any ideas on what stamps to use ?
 Thanks for your time....

Yamaha used a unique font style. All the stamping tools I've seen are different. Keep the old crankcase if you need to prove that the change was due to a repair. And the metal is hard and brittle. Be careful.

I've seen a few bikes that are blank.

Did not realize that about the font but not surprised I guess.

Is the concern regarding the case being stamped due to a full-blown restoration taking place, or just doing it "because", or a desire to be thorough? As registration is done by the VIN on the frame not having the engine case stamped should not affect that, and as been pointed out, you do have the old damaged(?) case half.

Some states require an engine number on a motorcycle title, not just the frame number.  (California is one of them)

If your state doesn't, just leave it blank.  If your state does require an engine number you might need to be careful and/or get them to assign an engine number with an ugly tag they install. 

I had a friend when I lived in California that replaced cases on a wrecked bike and he stamped the original number in the new case.  When he went to get it titled, he got flagged to have VIN inspected at CHP.  Officer said it was "illegal" for him to have stamped the number himself and they assigned a new number to the engine with an ugly blue tag over the number he stamped.  This was a newer bike, not sure they'd care or notice on an old bike where stampings were wonky to begin with.