Does anyone know where to buy a 12pt socket bit to remove 12pt bolt on side of engine case? Is believe it's 13mm.

It might be the one on Tangerine Racing's transmission/axle page. Not sure but you could email them...

Hine62

This question has been answered a hundred times. There are 4 sizes in a package

from your flaps. They are also called triple square.

There's no need to buy a special tool for the triple square bolt on the side of the engine case.
A 3/8" socket extension is a perfect fit.

Make sure you buy a good quality bit. The cheaper ones can be soft and not quite the right shape at the end. I found a good one at NAPA after damaging a cheap one from Auto Zone.

Socket extension has worked for me for nearly 30 years