Top work half, maybe 3/4 of it with a variety or two that you know you like. That way you can enjoy mangoes while you wait for the rest of it to produce fruit (my tree from seed took 12 years to finally flower). Grafting to a mature, healthy tree is as easy as it gets and the growth can be nothing short of explosive. If you get it done soon, you could be eating mangoes from the tree in as little as 2 years from now. I'd bet the seed tree flowers for you before too long, but why wait. If you absolutely love the fruit from the seed tree, you can always cut off whatever you grafted in and let the tree regrow.