Lew pointed you in the right direction for 5 x 4.5 aluminium replacement wheels. Some of the aluminum wheels on that site have ratings as high a 2150 lbs at 60 psi. I don't understand why a wheel would have a weight rating for a particular psi. Perhaps they mean that the wheels are rated at 2150 lbs and are robust enough to handle tire pressure up to 60 psi. Perhaps someone more knowledgable of wheel rating could weigh in here. Anyway, If you wanted to go with the higher rated tire like a 225/75R15 the aluminum rims would make more sense since they are closer to the load rating of the tire.
I would point out however, that there have been a number of posts on this forum that suggest that a tire with too high a load rating inflated to the pressure recommended on the tire can be harmful to the trailer. Since the tire is an integral part of the trailer's suspension system, it seems possible that an over rated tire inflated to the full pressure could in effect stiffen the "suspension" and increase the shock impact on the trailer from pot holes and other irregularities in the road.
I suppose it would be possible to underinflate a higher load rated tire but that would seem to introduce its own set of problems. I like the retro look of OEM steel wheels especially painted black with the chrome style hub caps that came with the original so I am planning to stay with the system that came with the trailer. I also noticed that the 1967
Airstream Manual recommends that the 7.75x15 tires in the Caravel be kept inflated to 45 psi. The recommended inflation pressure for the ST205/75R15 is 45 to 50 psi depending on the manufacture. This suggest that these tires might have the same "resiliency" as the 7.75x15 that came with the trailer and perhaps work better with the torsion suspension to maintain the same ride characteristics as the original package from the factory.
Anyway, I think I have beaten this horse to death and I see it is now about 1:30 am Sunday in Greenwich so I don't expect you'll be checking this thread anytime soon. Tires are always a fun topic on the forum let us know what brand and size you finally decide on.