You can get rebuild parts anywhere. Get from Nissan or Rockauto.com. Just get what you need. Dissasemble and measure everything you can almost always use your 1200 existing pistons.

How to install? Just like rebuilding a V8, nearly identical. Get a 1200 repair manual and it will have all the information. Be very careful, measure the rod journals before you buy the parts to make sure it is rebuildable. If they are bad, it will be cheaper to swap in another engine like an A14 -- a lot more power and less expensive than a rebuild/maching a bad crankshaft.