2002 RM 250 Spark Plug Help

I recently bought a 2002 RM 250 that runs good but looked to be neglected for a good while. I paid $800 for it as a little winter project. I pretty much replaced or rebuilt the important things such as suspension, bearings, sprockets, chain, bars, levers, and a few other things. The bike ran decent but seemed to run out of fuel or spark when you open the throttle fully. So I figured I would replace the spark plug just to eliminate that as a problem. This is where my issue comes. I bought a NGK BR8EG (this is what my local shop gave me and said the bike called for stock). I got the NGK plug home and immediately realized it was longer then the plug the previous owner had in the bike. I figured the previous owner didn't know what he was doing and put the shorter plug in the bike. I was kind of happy because I figured the shorter plug is why the bike was running like crap. So I put the NGK plug in the bike and slowly turned the engine over and everything seemed fine and the piston did not seem to hit the plug. So I fire the bike up and it barely runs (will idle for a few seconds but immediately stalls). After removing the new longer plug I found that it was smashed shut. So I reinstalled the old "shorter" plug and adjusted the power valve a bit and now the bike runs great. I was immediately mad at the shop for giving me the wrong spark plug but after doing some research online it seems that the RM 250 does call for a BR8EG. I suppose my question would be is this the correct plug if so why would it have gotten smashed shut. Is there a possibility that an aftermarket part such as a high compression head or some kind of domed piston would cause the engine to require a shorter then stock plug. Sorry for the lengthy post and if I sound like a noob (I am new to these big two strokes). Thanks for any info.