the nismo N! oil pump would be worth looking into also a larger radiator. the sump issue sydney kid refered to is a big one a nubmer of people have had this problem with oil starvation. and increase in fuel pressure should see fuelling increased. for pistons nismo make n1 pistons which are the ones you refer to you can get them from nissan here I believe
make sure when the motor is apart all the oil squirters are cleaned out properly some people put a restrictor in the oil gallerier but this might be only for big hp pplications not sure.
check out http://www.reimax.com/product1.htm they have alot of bits for group a racing I would also talk to the guys from Just jap they have built an r32 for targa and they might point you in the right direction.
cheers
meggala