Although I admire the early engineering in old sewing machines I don't particularly want to own one so I understand your computerized sewing machine delimma. This isn't a mechanical vs. computerized issue. It is basically what is the most efficient, cost effective way to achieve your sewing wishes.
I think your decision comes down to several factors: is the replacement motherboard refurbished or new, is the motherboard the only electronic part (if so replacement might be acceptable), how much is the machine worth in working condition, how much would it cost to repair, how much a new machine costs... The engineer comes out in me when it comes down to things like this. I know this is a huge decision for most of us. I like to get the most versatility out of any machine I own.
Did your machine go down because of age or was it caused by something insurance covered? If it was something like a power surge or lightning your homeowners policy might cover it (mine does).