Gunk and Jizer aren’t that cheap.
If you are still at the dirty engine stage, then neat Flash liquid (the stuff that’s almost fluorescent green) sprayed on, and then left for a little while. Then you work it in with a budget washing up brush from Tesco’s. Leave it and then rinse it off with one of those cheap plant spray things with nothing other then warm water in it. Repeat as necessary.
I only use Jizer for the last few stubborn bits.
It’s a lot cheaper, it smells nicer and its far more environmentally sound than just Jizer from square 1. A rinse of water/washing soda also works well, and is really cheap to make up. As long as it’s rinsed off thoroughly afterwards, shouldn’t be any problems.
I don’t know anything about painting aluminium.
My favoured method of painting engines is QD90. Its ok up to 150 deg c. Japlac fades very quickly IMHO, and I have had problems in the past with so called engine enamels.
As an aside I got a quote to clean the engine parts from my spare 24V senator engine. This is just a bare cylinder head, block, pistons, crank, rods and sump. All completely stripped down into separate constituent parts. £180+VAT. That’s a lot of Jizer, pipe cleaners and shop towels!

Morris Oils sell a powerful degreaser that we used recently with good results

I use Autosmart G101 mixed at about 8:1