Hi Rick and welcome,
no airforce memories here, I was 3rd generation on the civilian side.
My dad built Bristol Bulldogs in the 1930s my Pa-in-Law built Bristol Beaufighters in the 1940s and I (and 10,000 other guys) built the Concorde in the 1960s.
Bikewise I was a Velocette rider in the UK and a Triumph & Honda rider in Canada until I bought my NOS 1984 XS650 in 1986 because it looked like a Matchless.
So back to your XS650. 1981 is a good year even if it lacks the 1980's hydraulic rear brake it does have the electronic ignition so there's no points or mechanical advance mechanism to worry about.
Which means there's also nothing except the ends of the camshaft under the points covers so don't have a WTF? moment when you remove them.
And DON'T drink the PMA koolade. The stock system is better.
XS650 parts swap like LEGO. Any drum brake rear and any post '73 front wheel drops right in.
All Special gas tanks and post '72 (1 gallon larger) Standard gas tanks interchange.
1980 & newer "lift off" seats interchange but they don't look the same, the newer the year, the taller the hump.
Thanky, good info!