UPDATE: Y'all boys are great! Thanks a lot. Yesterday, I cut some of the 2x4s and plywood off the frame, and that thing is really strong! Steel must have been Pennsylvania made

. Even the plywood and 2x4s are birch, and looked great! So, I have decided to go ahead with this project. Also, it will be no problem getting it registered. And, I can make three more trailers from the frame - giving me the 18' trailer I want, and three 10' trailers to sell. They will be made right! My welder/machinist nephew will do most of the work for me, he does this kind of thing all day in his shop, so I will have good products - he is meticulous in his work.
I looked at new trailers yesterday, and compared to what we can make, they are not so good, and $1,795, no brakes (don't know yet if I will need brakes on mine). So, I will probably buy new hubs, wheel, tires for the present axles (if they will fit), then get new axles, etc. to build the other three trailers, probably single axle for those. Mine will be tandem axle, I'll get new axles too if the present ones won't work.
It will take me some time, I have other projects in higher priority, but I will post photos as I go along...
Thanks, nebu, for that Marshalco link - I checked them out, and they have the present tire sizes. I want new wheels, etc. though, so might go with new hubs and all.