You have an S90K engine if it has a square sand-cast head. I have an S90K as well, and my engine has the carb/spraybar arrangement that the G2300 does. Mine does not have a retaining nut on the needle valve, so perhaps your engine is newer than mine is. I'm not an ST expert, but I've rifled through enough threads to have a good idea. I do not know which needle will fit your engine, the listing said ST, MVVS, Como I think. They may all use the same thread/taper. If it were me, I'd use the spraybar setup from the G2300. I have an S29 with the bent needle and retaining nut and my needle valve spews fuel like mad when running over 1/2 throttle. My S90K needle valve does not do this. The S29 made a mess on my test stand with the leaky needle valve. But for $7 plus postage, its worth trying the bent needles.

Many parts from the G90 do NOT interchange with the S90/S90K engines. The cylinder liner is the same between the G90 and S90, however the piston, ring, and conrods are different. The S90/S90K have a removable front housing that can be rotated to fire the engine clockwise. The G90 is a 1-piece crankcase. The cylinder head and backplates are the same I think, and the carbs are really similar. The S90K has a milled spraybar, the new G90's have a round spraybar. The S90K is undoubtedly a stronger engine than the G90 based on what other G90 owners have said. My S90K has about a gallon on it, and turns an APC 13x6 at over 13,500rpm on just 5% nitro. On FAI fuel it loses about 800rpm but has a much snappier exhaust note.

The S series engines are designed for low nitro. Don't use more than 5% or you will have issues. My S90K is somewhat intolerant of nitro - had a hard time getting it to run reliably on 5% nitro. It now tolerates it but just barely.