I'm putting in JDM type S pistons, I was wondering if I need to replace the main bearings and the connecting rod bearings? Or can I just use the original ones?

 

If I were you and I had the motor apart like that I would replace everything. Mains and rod bearing, oil pump, water pump t-belt maybe rebuild the head with a 5 angle valve job cost about 150 for whole rebuild on head. Just buy a head set kit at the local parts supplier (about 150) and replace everything in the kit too. Get the kit before you take it to the machine shop they will need some of the parts in it.