With enough advantage could you craft a brawn +1 arm prosthetic with a repulsor fist using the inbuilt weapon crafting bonus?

I don't see why not.

All it says is a weapon under encumbrance 2 but repulsor fist has no encumbrance....

I'd allow it for 2 reasons.

1. Precisely because the Repulsor fist is designed to be inbuilt anyway since it's a cybernetic.

2. It's completely sh*tty compared to a whole host of other options that could be installed.