Which crank to go 175mm?
 

I have a 20 year old Suntour drive train and it's a 170mm. It works fine but after riding my other bike with 175mm I know that I prefer that length.

Any ideas on what to replace it with that isn't so new it won't work?
It has a Suntour BB but I don't know what kind of taper it has.

Also the existing crank has 42/53 rings so I am guessing a replacement would have 39/52 rings. Will I need a new FD to handle that?
Hmmm, and how would 39/52 work with my existing 7 speed freewheel as far as ratios? Maybe I could find a 175mm crank and put my existing rings on it...

i'd think most any square taper double crank in 175 will bolt right up. I'd also expect that the same front derailleur that shifts between 53-42 will also shift between 52/39. That's not much of a difference. Heck, Ive used derailleur's meant for doubles to shift triples, so I'm sure what you'd like to do is fine.

Post a swap of your 170mm Suntour for a like 175mm in the ISO/WTB thread. You never know.

Siguno makes square taper cranks. Try the Quality Catalog at your LBS. They aren't expensive.