I havent heard any negative on a BB kit, however the very nature reduces longevity. Im slightly concerned with the head bolts. I had a xr600 with a 660 BB kit (10% over bore same as a 400 to 440) and it pulled the head off.
The big suck with my bike is I need a new head, thats $600 for a blank head. Im not reusing any parts from the seized head.
By rebuild I mean the kits they sell on line, theyre mostly bearings and gaskets, top end comes with a new piston...I dont think I need to go hog wild on a head, nothing fancy just stock parts aside from cams.
You cant fix stupid, but you can sedate it.