just had this problem on a friends bike after a late night recovery call I couldn't fix roadside so threw it in my garage - 99 derbi gpr50r
symptoms and quick checks...
the spark was there, carb was cleaned out, felt like compression when finger over plug hole but nowhere near enough, wouldn't even fire with carb cleaner poured into combustion chamber, but occasionally would start bump starting on a hill (but ran rough and sounded wrong ooh so wrong)
you might be able to do this in bike but I dropped the engine for fun.
pull off the head then the barrel from the piston push out the gudgeon pin and remove piston for best view
- there was scoring on the barrel and the ring had 'notched' causing a stuck ring on one side and was flush with the piston (see the problem yet)
a new piston (£13) and a little wet and dry paper would work but no guarantees on how long for (I don't advise this)(+£13 for gaskets = £26)
a new barrel piston gaskets and all the little extra bits start at £50 on fleabay but we spent £65 to get a slightly better quality one
you are open to decide if you want to go big bore kit as this is the best time to make the decision - up too you.
search google for carl salter to get manuals for torque numbers etc - I advise to buy beg borrow but not steal a torque wrench and doing it right - and add some copper slip on your nuts (as in nuts and bolts) doing it right now saves doing it wrong later and MOST IMPORTANT COVER NEW PISTON AND BARREL IN NICE NEW 2T OIL WHILE ASEMBLING TO EASE WEARING IN AND MAKE SURE YOUR OIL PUMP IS WORKING AND HOSE ON CORRECTLY AS THIS IS OFTEN THE CAUSE OF THE PROBLEM
SHE WILL SMOKE A LOT TO START WITH BUT DONT WORRY ABOUT IT TOO MUCH IT IS A STROKER AFTER ALL.
-HAVE FUN, OZZY