I got a Fatso motor in my 88 FXSTC. No idea how many miles on it cause I converted to an aftermarket gas tank years ago and got rid of the dash, speedo and such. Motor runs strong, breaks the ass end loose like nothing. I've had that motor probably 7-8 years, it was built by a company called Milwaukee Performance on top of cases from a merch 120.
No motor problems, but all the torque kept cracking my primary case, right around the stator, compensator sprocket area. Fixed that with a carlini torque arm(after 2 or 3 inner primary's) then I sheared that silly little keyway off my mainshaft that holds the clutch in place,prolly 3 times b4 I finally was able to upgrage to a splined, later style clutch setup. LOVE the motor though, just that cheesy 88 drivetrain could not handle it.