I located a Ducati 2004 Monster 620 motor locally that I was able to pick up pretty cheaply. I got the engine, swingarm, throttle bodies, airbox, ignition components, clutch line and master cylinder, and instruments for $230. Not a bad deal at all.
I have no idea how many miles are on the engine.
The front mounting boss at the base of the front cylinder is broken, which is a real bummer. I might do something akin to a “cripple triple” type build just to get it running, then do a proper supermono conversion to a more interesting choice, such as a 1000DS engine. (Dual spark 500 single? Yes, please!) For right now, however, I decided a cheap running engine with a broken mount was a step beyond what I had: a set of empty cases with a broken mount.