Tbh I wouldn't worry in the least about a cracked head unless you're going to modify it and tune it. In my experience the failure point it the rocker pad wearing through and the cam hardening flaking off, but even then they seem to keep on running...
Hmmm, wonder if an XBR lump would fit into swmbo's nrs chassis....

All opinions expressed are of course mine and your mileage or opinions may vary....

Edited to add, the NSR has about 9.25cm across the frame rails, anyone know the width of an XBR head...?