Well, the great thing about these older motors is that they are simple, simple, simple. There are only a number of components that could fail.
It would be easier to debug if you know if it was a fuel or spark issue. Judging by your last comment, seems like it may be a fuel issue. Easiest think to do is check all the filters... the copper filter in the carb bowl, and the water seperator filter should be checked. But my best guess is in the carb itself. It sounds ilke ... posssibly... the fuel pump can keep the bowls full at mid throttle, but not at WOT. At WOT your motor is sucking the carb bowls dry, and waiting for the fuel pump to refill. I would bank on a bad fuel pump, or clogged filters.