The 1979 mariner 21' was rated at 150 (125 for the 18'), with a dry hull weight of 900 pounds. I'm a firm believer in maxing out horsepower whenever possible, but I understand that it isn't always possible.
At a minimum I would think 70/75 would be needed for that hull and it likely would only work with light loads. 90 would be ok, 115 even better.
I've run 60 and 70's on flat bottom 2060 work boats that have weights in that range and been favorably impressed with the performance. Yours being a V will require more HP. I would think the 90 should be a minimum. and would seriously look at a 115 if funds allow.