Frame # is XS1B. Engine no is odd. 8650 looks to much like S650 with the S looking like an 8 and this could have come from the factory like this. In a case of a replacement set of cases they would be stamped S650 with the Vin no is missing. Did hear about engine cases getting replaced and came fro the Factory with no Vin #'s stamped on them. You could stamp your frame no on the engine if you do but do some research on pics and make sure the stamp looks like factory.
Dose the motor have a 3 bolt tappet/rocker cover covering the left side tappet/. It has to be a 3 bolt cover and no starter motor, (motor hole), on the engine case for it to be period correct. These early motors did have a reputation for stretching the little ends on the rods and breaking. This could result in a rod going through the crank case. Hence the replacement case on yours if this ever happened. not trying to scare you it is a possibility on why the engine case doesn't have a vin #
XS1B's did come out in Candy green so it could be colour correct. Tank single green stripe is XS1. XS1B had double stripes
XS1 and 1B side by side, Candy Green and Candy Orange
View attachment 197392
XS1B candy green
View attachment 197393View attachment 197394
Now you mention the bike Has KPH speedo and a Vietnam vet imported the bike into the US. As much information about this and from where will help. What year it was imported as well if you can well.
There are a couple of guys on here who know these early XS's real well. TXXS is one so a PM to him if he doesn't answer on here may help you with more info.
As for the value. Defiantly worth good money. XS1's are the top of the lst but good XS1B's are the next best thing