You can't cut with a mig/tig but:
You can't weld aluminum with gas (you can with tig )
Steep learning curve to gas weld
Can't weld sheet metal with gas, but you could braze
Your better off to do all your cutting with a blade saw. Either a sawzall or for fabricating small pieces use a band saw.
Clean straight cuts are much easier to deal with.
I've never welded aluminum with mine, but I was told you can just get a role of aluminum wire and weld some aluminum. Although, I have never done it, or needed to. Not much aluminum other then the motor on these old bikes.
Not much aluminum welding to be done on older motorcycles.
Mig welding is way easier to learn then trying to learn with torches. Just a little practice on some scrap steel and you will be on your way.
It is also way more versatile and easier to use then a stick welder,(have one of those also).
I wish someone would have told me I couldn't weld sheet metal with my wire feed before fabricated my tail light assembly, battery box and and some other items before I did it.
Anyway, not trying to be disagreeable, but I have found the wire feed welder to be the best and easiest way to fasten to pieces of metal together in a neat and very Strong manner.