If you upgraded to a SHA15:15 I hope you also changed out the intake from the stock 13,8mm ID intake or you may just be compounding the issue by adding a restriction after the carb.
Thinking that you might have smeared the rings when the union between intake and carb failed. Needs a bit more TLC and freshening up the top end is a goodly thing to do for a old old bike.
Get a upper gasket set (head, base, exhause, intake) and a set of rings (FG type) pull the jug and check that the poor old old rings are not stuck in the grooves. Chances the rings are tired anyhow over the years so goodly time to fresh them up. Gap the rings before installing them to the piston (0,15-0,16mm), they do not come pre-gapped (close but they will be a wee bit too tight)
If there is any slight smear of aluminium on the chrome cylinder wall you can use a Qtip to carefully apply a little Drano to the smear ONLY (it WILL eat aluminium quickly so careful not to get it anywhere else) then buff out the now flaky smear with a clean rag.
Decoke the head and the exhause tract while you have the jug off. Use some oven cleaner inside the exhause (dont get any on the aluminium, some are very harsh on aluminium). Follow the instruction on the can and flush out the old burned on coke with a hose. Maybe you have to do this twice if it is very coked up. Take the end cap off the silencer, remove the body sleeve and clean out the baffle holes and the holes in the piccolo tube exit on the cap. Use a old drill bit in your hand as a reamer for that, also clean out the exit pipe with the drill bit.
Reassemble after cleaning everything, use some two stroke oil as assembly lube so there is no scratching.
IMPORTANT you MUST use a new crush gasket on the exhause big nut when reassembling AND you must check it is right tight AND recheck it when the engine is hot hot so that it does not shake loose from normal engine vibrations. If the nut shakes loose it will wear out the threads in the cylinder rather quickly.