Hello, and congratulations on your project tritoon.
I would recommend talking to a competent local aluminum welder. If you hope to do a patch repair, speaking to someone with experience doing such work (and the one you’d potentially hire to complete it) I think would be a smart move. Good aluminum welders can really do a lot of things. I’ve heard of pretty major damage being repaired effectively in this manner. We even had a local aluminum welder handle warranty repairs on our port side pontoon that had many leaks along some welds. It’s been good as new ever since.
Buying a replacement pontoon would be incredibly cost prohibitive for an older project boat. I hear numbers like $7k and up (but have never looked into it myself) just for the pontoon, let along the retrofit work/labor.
Finally, where is the impressive dent located? Can you post pictures of it on here?
If it’s literally on the very front of the center pontoon, that’s going to be out of the water when up on plan and a total non-factor in regards to performance. Thus, if you can economically get a patch repair done, I can appreciate doing so for aesthetic reasons. But if the alternative is a full pontoon replacement, that would think that it would be a very pricey job simply for cosmetic purposes on a project boat.
Just my 2 cents really. No matter what, I hope the project goes well, and you are very happy with what you end up doing with it. Pictures if you want to share your project with the group. Some very smart DIY types in here that might be able to share insights & advice (not me on the DIY part, but others for sure).