I rarely bother painting frames these days. Almost everything you can apply at home is softer and weaker than the factory paint. It'll scuff, chip and scratch more easily. And painting on bare aluminium is tricky in itself. You need to use an etching primer to have a decent chance of the paint not starting to peel pretty much immediately.

My first thought would be to leave it alone. Bikes are meant to be used. Let it wear its battle scars.
2nd thought, if the old paint isnt flaking, leave it. Scuff and clean and use it as primer/base layer.
3rd thought, media blast and Powder coat. If you're really set on getting a good result. Order up some new decals while you're at it.