The good news is that you can get 3rd party ram for the 4200 (it will accept 1 additional stick of up to 32MB for 40MB total), and you can get replacement hard drives. The bad news is that new replacements will run you close to $500.
Personally I don't think it would be worth it to spend that kind of money on that old a laptop, but you can try crusing E-bay and other auction houses, you might be able to pick up good used parts to rebuild your laptop at a reasonable price. You really don't want to have less than 40MB memory and nowadays even that is marginal at best. If you can do it on the cheap I'd go for it.
To give an example why I'd think twice about getting new memory and disk, for about what you'd pay for 32MB memory and a new 4-6GB hard drive you could pick up a good used Dell Latitude CP with CD/Floppy/64M/4GB disk with 12" color active matrix display.
Just my $0.02.