Given sufficient time and money (mostly money) anything is possible. How large is your budget?
Cutting the roof off a 1973 Beetle is not the best way to get a convertible Beetle, and will cost WAY more than just buying a convertible in the first place.
If you want a convertible Super Beetle, shop around until you find one. You’ll never be able to make one as good as a real one.
Are you planning to use this car as a daily driver? Have you ever driven a Beetle of this vintage? I can’t imagine trying to keep up with modern traffic in a car like this.