rebulding my 125 mx 1981 will not rev oveer 5000rpm when i bought it i was told it had a rebore any ideas plz

Have you tried cleaning the carb? get a strand of wire wool and poke through the jets to unblock anything obstructing the flow then blow through them. this may not fix your problem but I do it every time I get a new bike do this first.

Is your choke getting stuck? try opening the carb and loosing it up and lubricating it it may be stuck on.

Is it running on pre mix? if so look up the recommended mixture ratio then flush the tank and refill with fresh oil and petrol ant the factory recommended ratio you may have too much oil in it.

Also let the bike fully warm up and adjust the mixture screw on the carb until it will freely rev over 5k.

is your exhaust new ? with 2 strokes especially ones that are older the exhaust will become coked with carbon try taking the exhaust off and leaving it upside down over night then flush it through with exhaust cleaner when you have done that get a screwdriver and LIGHTLY scrape off any stuck on carbon.

I agree with dogwafter sound advice I had a similar problem with my aerox 2 stroke and stripping and gleaning out the carb did wonders for performance.