I have a clarifying question and something to contemplate. Are you considering replacing the canvas tent on the pop top or are you talking about completely altering your vehicle radically by removing the existing pop top and installing a custom and permanent raised hard top? If it's the latter, you'd end up spending A LOT of money and you'll need to end up painting the entire van to have it look like it wasn't an afterthought. If it's really that important to have the permanent high top, my advice would be to sell your van and buy what you really want. It will be way less expensive and you won't be destroying one of the very desirable pop tops that so many people like. I'll grant you that they aren't for everyone, but those of us who like them search to find them. We drove three hours with two drivers to find ours.
But you're saying it leaks. If you're convinced that you need to keep your current van for some reason, then just get the pop top fixed properly. Buy new seals, take it to a fiberglass professional, etc. That will be way less expensive than the other route. Those are my thoughts. I've renovated 27 pop top campers of different manufacturers. Hopefully this gives you something to consider. I wish you the best in your project.