I need help !!

I recently bought a rebuilt 1992 kx 250 ...fired first bike idled fine took for short run through all gears seemed good so I bought it ..next day i fired it right up and went maybe 1 mile down backroad and it locked up .....seems piston was stuck ....pushed back home tore the top end off easy piston was fine didnt have any damage....so put it back on ...third kick fired right back up ...but now there is a extremely loud hard knock sound that scares the shit outta me as I dont want it to drop rod through case etc .....has v force 3 reads new ..new head new piston ...aftermarket clutch pro circuit pipe .....any ideas from you guys were j should be looking before I tear it down again be much appreciated