I would like to replace this Custom 52t/42t crank with a more of a touring crank like 48t something on my 83 Univega Sportour. Will it work with my 6 speed freewheel. Or, can I get smaller chain rings? Any suggestions. Will a mountain bike crank work?

Looks like a 110 BCD, in which case you can use a compact ring set, usually 50/36 or 50/34, pretty ubiquitous these days. If you really want the 48t, you should be able to source one for this bolt pattern without too much trouble.

You'll want to verify the BCD, though. If it's 130mm, the smallest it'll take is a 38t, and you'd likely have to search high and low for a 48t.

Your FD might look a little unsuited to the curve of a 48t ring, but it should work pretty well since FDs were a lot less fussy back in the day.

No dramas with your freewheel (I can't imagine what), except if it was me I'd ditch it for a 7s Hyperglide job.

Cranks don't care how many "speeds" you have in back so any road or MTB crank will work. A newer 9-speed or 10-speed crank may not be happy with your wider chain but otherwise, they will work. If you do change the crank, you will most likely need a new bottom bracket to match.

As best I can see from the photos, you have a crank with a 110 mm BCD (bolt circle diameter) so any chainring with that bolt circle will work. You can fit as small as a 33T chainring to that crank so you can regear it as you wish and chainrings in that bolt circle diameter are quite available.

Keeping your crank is definitely the go, much simpler (although chainrings cost nearly as much as entire cranksets). You'd just fit the rings, lower the FD and shorten the chain.

This is a nice old bike; I wouldn't replace anything that wasn't a wear item with non-original stuff, or it'd be spoiled.

+1) just change the chainrings themselves ,, a 48 34 is possible .. and EZ.

a TA 33t is overspending a bit..

You'll definitely want to verify the BCD: Sakae Ringyo made a bewildering variety of cranks marked as "Apex." While some have common BCDs like 110 or 130, others have unusual BCDs like 118mm. So, check the crank before you commit to any ring.