You could also do it on a mill I can think of at least 3 ways to do it on a mill.
Jig and a router would also work, your regular carbide wood bit will eat aluminum all day
Without taking any damage.
Make a box the piston will fit tightly in drill a cross hole to put your wrist pin in to hold the height,
So it doesn’t try and pull out of the box you get the Idea.
Think like a machinist if you have the tools at hand if not think like a furniture maker.
That will get the skirt cut down then just check the weight
