Modern Vespa

I found a transmission side of a p200 engine case I didn't know I had. How important is it to have a matching side case?
A- If it is important, why?
B- is it possible to find a matching one or is each case made to each other respectfully?

I believe the cases are cast then two halves bolted together. Then they are bored to take the crank bearings as a pair and stamped.

I'm guessing that you might get lucky and find 2 mis-matched halves that are close enough for the crank to run true. Or maybe not.

It'll usually work for a few thousand miles before the bearings start making a racket. Somethings longer but not usually.

I had a P200 engine with mismatched cases. I rebuilt it 2 times in under 4,000 miles. The crankshaft got so worn where the clutch bearing sits on it that I could pull the crankshaft through with my hand.

I tossed it all in the garbage.

with that being said, is each 1/2 half of an engine case only compatible with each other?

So when looking at the numbers stamped by the case mouth, my case is the only one in the world with that number?

pretty much

in addition to what has been stated on the crank I've run into two other issues with mis matched case halves. The bore where the cylinder spigot fits into the case was not round due the cases not being matched. Also the surface where the cylinder mates to the case, one side was taller with a large enough gap under the short side so it would not seal. Expensive lesson learned ruining a new pinasco kit. This was an extreme case of bad maybe not all mis matched scenarios are as problematic.

I had two mismatched cases put together... the crank bearinsg always whined and made noises right from the get go.

What a shame

I have a case where the flywheel side bearing journal has become oval. A friend (aluminium welder and lathe operator among other things) said that it was no big deal to weld the journal and re line-bore the cases. Don't know what that means in terms of time/money but new cases may be more expensive.