if you change the crank you'll have a 32 chainring with 21 cog will give you the same approximate gear ratio as if you only changed 1 cog on your cassette to a 28 tooth and left your cranks alone. i see no advantage to changing the crank to a mountain crank. this is why:

1. the cost of a crank and possibly a bottom bracket will far exceed the cost of 1 cog and would most likely exceed a new custom cassette from loosescrews.com.

2. you would be breaking up a really cool, original component group . this not only would decrease the value of the bike in my opinion, it just wouldn't look right to me.

3. your front derailleur might have problems with that setup. the derailleur cage follows the contour of the chainrings to help it shift properly.

4. i'm sure there is a 4, but i can't think of it right now.

i'm sure you are pretty confused by now. look, keep it simple. change the cassette gearing. if you get a 28 tooth large cog and put it on your existing cassette you can most likely keep your chain. if you replace the entire cassette with a new one (28 tooth large), you should also get a new chain. this may be all you need to do, try it and find out. if you need lower gearing, you can replace the inner chainring to a 39 tooth. it will all work perfectly, i had a bike with the exact same components and it all worked.