Hi Mate
Welcome to the forum .Always good to get more aussies here .
How does it run once started ? They do need a good battery to start .
Could be crankseals .

With your oil tank i would pull them of clean them out and fill back up . I run the oil injection on my bike and don`t put any oil in the tank if the pump works that is all that is needed .

You would be able to use a mc21 brake light spring

With your netral light I take it has got the mc18 wiring with pgm1 or pgm2 ?
the mc21 has a gear possition sensor and the mc18 has a neutral switch you should be able to find the neutral wire in the possition sensor and tap into the netral wire in the loom .