SunTour XC Pro MD front derailleur cross compatibility
 

It's a front derailleur, it doesn't care who makes the shifters or indexing, and it's too bothered about the speed either (at the back, it does at the front) + Suntour only lasted till 7 speed, so it's the same as you currently have.

I disagree with the bolded part. Ft indexing does want specific and matching components if you want best function. Otherwise matching cage shape and ring sizes is a want too. Running a ft der made for a 52/53 ring on a 42 ring makes for a lot of open ring/cage gap where it counts. Much like having your guide pulley too far from the rear cog's underside. Andy

What is missing here is information about the shifter you are using
Crankset, check. Derailleur, check. Shifter? No idea

A Suntour XC Pro MD front derailleur was designed for a 42/32/20 so it should work quite fine with this chainset. The original XC Pro front shifter was a non-indexed friction so you may have some issues getting the trim right but should work.

Suntour XC Pro went to 8 speed the last year it was offered. The only change was the cassette and the shifter, the RD was the same. At least that's all I needed to change on my Hei Hei when I upgraded it to 8 speed.

FWIW: these are "Rapidfire" shifters, where the shifting is activated by clicking on a lever or trigger under the handlebar. What is normally called a "thumbshifter" is a single lever mounted above the handlebar. It's a piece of bicycle mechanic trivia that might help, particularly with surly old farts like me.

Now, that SunTour XC Pro MD derailleur was never designed with index shifting in mind. The MD means "Microdrive" so the cage is shaped to fit a small (42-tooth) front chainring, so that might work in your favor. The sticky point is the cable-pull-to-cage-movement ratio. This affects everything about the shift and I frankly don't know if it will work. Don't get me wrong... if it were my bike I'd try it out and see what happened. I'd set it up so the cage was centered over the chain when the shifter was in the middle click. If it made the shifts to the big and small chainrings from there, it might work. Test rides and tweaks would certainly be needed- front derailleur adjustment takes a lot of patience and a chunk of understanding what's supposed to happen versus what's really happening.

Your original front derailleur was a Shimano Alivio (the big "A"). It's a sturdy, reliable unit. I'm surprised it didn't fit the chainring... the cage should fit a 42 or 44 tooth chainring well.

Like Jeff Wills said, it might be worth trying, to see what happens in terms of the cage travel over the chainrings.

I had a road Cyclone front derailer that worked perfectly with an Alivio Rapidfire triple shifter and SR Apex triple crankset, sothis might also work.

I would set the cable tension with the chain on the middle ring, shifter in it's middle "2" position, with the chain on the largest rear cog, and with the chain then nearly rubbing the front derailer inner cage plate as I pedaled under power.
Then I would see how the shifting was to the other rings, and how the chain cleared the cage without rubbing in the useful gears (not including the smallest chainring and smallest cogs at the same time).

On some front and rear derailers, one can move the cable anchoring to the opposite side of the pinch bolt in order to modify the derailer's travel (actuation ratio).