If you have a mc18 crank with the loose seal in the middle then the pin is 24mm and I think I saw a rod that may work . It all depends on the space that you have for the front cylinder . The Nsr 150 rod is 5mm longer than a stock 250 rod and I assume it is the same as the nsr125 .
For mc21 bearings you need to talk to Andy .
Are those barrels nikasiled ? If it was me I would go bigger with the piston kits available or go the 300 route as there is history and set up info on them . The Polini stuff has not been spoken of here so everything will be new and untested . So for 308 headaches rather go all out or do the common 300 conversion .

I have a list of rods and pistons that may work but I have to find it first .

Good luck