jgedwa

Give single-speed another thought. Depending on how you will use it, you can probably gear it so that you do not have to mash too hard for your knees to stand.

Does it have horizontal rear drop-outs? If so, maybe all you will need is to find a rear wheel to make it work. If not, you will have to use a tensioner (or a dummy rear DeR) to keep the chain tight. With luck, the crank has bolt on chainrings, so you can unbolt all but the one you want to use.

I think you might find that the parts to bring this thing back to life as a multi-geared bike will be pricey unless you happen to have a parts pile in your basement to sort through.

Some Hardrocks of that era have horizontal drops, but have most of the dropout filled in to keep the wheel in-line. This gives you little or no play to tension the chain. I have successfully filed out that extra material to give me a good long dropout to work with. If this is Greek to you, take a picture of the dropout on the right side of the bike and post it here and we can talk you through it.

Of course, you could also go fixed gear too. My favorite bike is a Hardrock with filed dropouts that I run as a fixed gear with street tires. It is a tough and simple urban assualt bike. And cheap too.

jim