How much is too much to Restore my Z50?

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I have a 1969 Z50A-k1. I was debating on restoring it to its former glory for my son. But I really do not know what the value is (or would be) on this bike. To get it looking and running good it needs cosmetic attention and a good engine overhaul. My son will outgrow it soon though and then I would sell it. I want to be able to make back the money I invest restoring it. But......What is the value on my bike? I have no idea. Would really appreciate some expert advise on how much to spend.