1) where's your timing?
Sometimes they were intentionally restricted by running ludicrous advanced timing, like 30 deg BTDC. Also sachs are fairly notorious for shearing the woodruff keys and slipping timing. I blame how fun it is to rip coaster brakes and the not-instant reaction from the clutch to disengage maybe
2) what's the ignition?
my 2-speed D balboa last spring got progressively slower and slower on top speed, had the cheap puch CDI, turned out the timing pickup or something was shitty and it basically just turned off higher rpms. People complain about that happening with those CDIs on puch too. If stock ignition, shitty points, tiny gap or shitty condenser or even a bad lobe on the inside of the flywheel might make it break up at higher speeds.
3) 4 strokey sound at top speeds can also happen if decomps be leaky or small compression leaks. I don't actually know why but I had a leaky decomp that sounded dead ass exactly like 4 stroking, and only appeared at high speed. Stock D w slip-on pipe, bike ran fine up to like 30 then sputter wall sounded just like it was 4 stroking, played all around with jetting and happened same even when the jetting was definitely lean. Then I pulled out the decomp and plugged it, rejetted and she was mid-high 30's