Sorry, that was a bit silly.

Stroking the crank means to change the actual stroke (the offset) of the crank pin.

Generally you would make the offset larger. So on the NSR the stroke is 54.5mm, so the offset from centre is 54.5/2=27.25mm. Move this offset to say 28mm and you increase the stroke by double that. The stroke would be 56mm, so you could say you've 'stroked' it a mill and a half. This extra stroke gives the engine more capacity.

Normal NSR: 54mm x 54.5mm = 124.82cc
Stroked 1.5mm: 54 x 56 = 128.25cc

WOW!!

Stroking can be done an a couple of ways. Rebore the crank pin hole off centre at the desired offset and fit a larger pin. Or weld up the hole and rebore at the desired offset. The NSR could possibly be done by welding the crank pin and regrinding at the new offset, then welding and reboring the out crank wheel holes at the new offset.

You'll probably have to replace the conrod with a different length rod or you may have the piston hitting the crank. A longer rod reduces the crank angle through the stroke. Oh there's lots of things. Better off searching the net to find out more about it.

Try here: http://victorylibrary.com/mopar/rod-tech-c.htm