Hi, I have a Gen 1 Alpha One with a good sized rock scar on the torpedo of the lower unit. I'd like to fill and paint it. Is this done using normal aluminum fill & fair methods?
And while I am at it would I be doing any good to knife edge the leading &/or trailing edges of the lower? What about building up a more pointed nose on the torpedo? Is it worth the effort?

Describe what you have in mind as standard aluminum fill method. I would not add a torpedo, you will actually increase the hydrodynamic drag with the added surface area, I would recommend a standard straight leading and trailing edge for the drive. If you are welding the fill area, be mindful of heat build up and case warpage. Good luck.

I was thinking of using one of those 2 part mixes that catalyze and form an alloy. The scrape is rough enough that I'd expect a good bond to form. There's lots of purchase available.
And no I'm not going to heat it.

A very good choice. I have had excellent resuslts using epoxy primer then base/clearcoat urethane final finish. The epoxy primer is critical, if the primer does not adhere the rest of the paint will come off. You can also use an activated single stage paint over the primer. Good luck.