If a bladder tank is available in a size that fits your current tank, I think that would be my first choice in a boat that has a difficult-to-remove original tank. In the past, I have had good results from tank liner products like Tank Kreem, but they require the tank to be removed for proper preparation & application of the product, so that is not an option here.
You may want to think about your past fuel usage to determine if the size of the original tank was a good fit for you or not. If your fuel was constantly 6-months old or more, &/or needed polishing more than once, then you might want to consider a smaller tank. If you were constantly packing the boat with jerry cans to stretch your range, then you might want to consider a bigger tank, or a second tank.