I know this isn't what you want to hear, and it's not what you asked, but if you had a trailer shop replace the axle, they know exactly what they put on it. Have them get another one exactly like it, move the original forward or back, and add the second. Put on some longer fenders and use it.
I've never understood the vitriol even the OEM steel

trailer gets, a la glorified shipping cradle. Yes it's light, but the boat is an integral part of the system, like a pole trailer (the logs or poles are what gives it strength - the long tube, if even present, just connects the two halves). My OEM trailer is too light for the boat (3500 lb gross), but the POs added a second axle, and it tow like a dream.
Oh, and don't be too impressed with the larger main beams and cross members of the typical aluminium trialer over steel trailers - aluminium has about 1/3 the stiffness (called Young's modulus), so members have to be larger. Not for strength, as aluminium can be made far stronger than common steels used for general work like buildings, bridges, and trailers, but for stiffness, which doesn't change when you make aluminium or steel alloys stronger. That's why bicycle frames use such large tubes when made with aluminium.
But it you're going to sell it no matter what, then I wouldn't pay more than 1/2 of new, and probably a fair bit less, as you can't always see the rot, like in the steel tongue tube - a common failure area on these trailers, from what I've seen here.