Are the side windows the old-style square jalousies? If so, they're considered a prize. They give the best ventilation, even in the rain. Are you sure they can't be rebuilt with new gaskets and parts? Member Ian G. rebuilt the jalousie side windows on an old Boler
and they came out beautifully. Look for his thread "It Started as a Kitchen Reno." In any case I would not use plexi on the side windows- you will want some ventilation in there. Worst case look for new windows in a slightly larger size and enlarge the opening to fit as Dave suggests.
Regarding the walls... Wish you hadn't taken out the Ensolite. It can almost always be saved. Absolutely do not use tar. A few people have used bedliner, but it only minimally addresses the three reasons for wall coverings: insulation, condensation, and sound-deadening. I agree with Dave that going with the current Scamp
combination of marine hull liner (known in these parts as "rat fur") over Reflectix foil-bubble insulation is your best bet. Again, you should be able to pull up some useful threads here, but if not, ask and someone will point you in the right direction.
Best of luck with your rebuild. Take it one step at a time, resist the temptation to shortcuts, and don't feel like you have to have everything done to enjoy camping in it.