Suzuki 2 Strokes

My all original (2nd owner) 1972 TS250 starts, runs and rides. Idles perfect. oil injection working. It is completety original. As far as I can tell the top end has never been apart. The only thing I have done to it is, new tires and tubes / spark plug. Fresh fluids. Very low miles. Shows about 2K. Recently it feels not very zippy, boggy, winds up ok but vibrates alot. After it runs for a couple of miles and you let off the throttle it goes, dong dong dong with a background sounding chattery snappy two smoke popping. Air leak? I have not torn into it yet. Maybe a leaking crank seal? smokes, but not a fogger, normal 2 smoke blue smoke. Crank bearing worn? arrghh...

Has it ever been run on premix without oil injection? Doing so will starve the left main bearing, you could pull the magneto and check the crank seal and if there is play in that bearing. I would pull the head and check for broken ring or play in piston and rod.
When you say it vibrates and chatters makes me think of mechanical issue.