To respond to your question, I would look at it the same way I look at car repairs. If the cost to repair equals more than 50% of it's current value, I'd try to replace it. I couldn't find any information about pricing in US for your machine, so I'd go with standard depreciation rate for sewing machines which is 5%/year. That would make it worth $320 if it were fully functional. So, I would spend no more than $160 total (inspection & repair) on it unless I simply didn't have the additional $160+ to get a new machine.
If you do upgrade to a new machine (or new-to-you machine), see if they won't still give you something for trade-in value of your current machine. Even though it doesn't run, the parts are still worth something.