Suzuki 2 Strokes

do i have to paint the upper case half or can i leave it bare soda blasted aluminum? what is the upsides of painting it? or is it merely cosmetic? what sort of paint to use too?

If you're going to paint it I'd advise a 2K SS silver (2-pack Single Shot) with some matte additive so it doesn't end up looking too glossy.
Single Shot 2K will enable you to paint straight on to the aluminium, no primer required. 2K means that it will be impervious to oils, fuel etc.
You need to thoroughly degrease and rough-up the surface... I use wet and dry and steel wool to get into tight areas.

Mind you I've just taken a look at my GT500 motor (not cleaned up yet) and it looks like it's unpainted bare aluminium.

Personally, I like the bare aluminum. To me it looks much more machine like. I might be wrong but I don't think any of the GT's had painted cases from the factory. If you want painted cases, buy a brand Y.

IIRC, on the 750, the top half was painted and the lower half was as cast.

The paint helps to seal the pores of the cast surface and makes it easier to keep clean. The downside is that paint tends to yellow over time.

My personal preference is vapor blast which leaves a really nice smooth finish, but no where near me offers that service that I know of.

I haven't found the right pic yet, but I could swear there was no painted upper case on my 77 barn find
Though not as good a vapor blast, these covers were glass bead blasted and powder coated with no gloss clear. I loved the look and they were silky smooth.