It's not a difficult rebuild. Not sure but if you got hold of an NSR125 service manual I suspect it's not that different, seeing as the NSR150 was based on the NSR125.
This you can find online I'm sure.
Are you looking to rebuild the engine? If it's a simple top end job then you could do worse than just follow the same procedure as fitting the NSR250 top end. Instructions are given in the workshop section of NSR-World.