If you do it yourself, just make sure to do your research on how to, and one word of advice would be to get real torsion bars. Do not use anything that could bend or break. Replacing them is really simple, just be safe!
I would open it up and make sure the wires are secure. Is there a starting capacitor? You could check that. Also, check to make sure it has been oiled. I had a cheapo compressor that would not turn over anymore - gave it to my Father in Law, and he added oil and it still works - 15 years later.
My Dad had the exact same set. Too bad I was a dip... kid and lost most of the sockets. I still have a few of them and one of the ratchets.
I vote to leave it as is. A great piece of history and memories!
https://www.harborfreight.com/power-tools/oscillating-tools/oscillating-multi-tool-68861.html
A friend of mine gave me one of these a couple years ago. Works great. PASS