1 - no, Jetts were budget forks at the time, and can't be upgraded
2 -As long as the fork is listed as suspension corrected, the length isn't an issue.
You most likely have a 1 1/8th inch headset (you need to measure to confirm) and if so realistically, you probably need to change to an ahead headset, as most forks aren't normally supplied as threaded today. This will require an new headset & stem. Additionally most forks now are disc brake only, ones with canti/v bosses are still around, but will need some searching.
If you have a 1" fork, getting a threaded / cant boss one will probably be easier.