1989 gpx 250 blown piston gasket - major edit- pics added

That's what i'd do, after i take the pistons off and clean them up a little as well as checking the head for warpage.

I spent 800 on pistons rings valves rebore gaskets- a rebuild above the crank only, a few months ago.
Found a zzr motor at mwb wreckers for 700 yesterday. I'm not buying it now. There's a motor for auction on ebay too.

If you decide to put it back together yourself, I too would recommend checking the head for warp by placing a steel ruler across the head and sighting any gap that might show between head and ruler on the machined surface as you move the ruler around. If it is perfectly flat it should be safe to assemble, but if there is warp it will need to be refaced at a machine shop first.

A brass brush is smaller, finer and gentler than a wire brush, about on par with a soft toothbrush. Wouldn't use it where something might fall into the crankcase, but in a parts "wash-trough", fine.

white smoke means burning coolant

reassemble replacing head gasket

a compression test before disassembly would have been beneficial , along with a cooling system pressure test ,

cool, all taken on board

re-assembled the bike and it wont start
its turning over and all, new spark plugs, leads are sparking (tested against the side of the engine)
im thinking its not getting fuel...as a guess. im at a loss though. i dont know where to start

does it sound like fuel is getting through?

it just sounds like the bike is turning over trying to start, with no ignition