When I looked at fixing up my buddies trailer, I was going to just cut off the fabricated walking beam, and weld in the full air suspension that came with the tandem axle set.
It looked like less work to me, to weld in the new suspension to the trailer, than try to do the axle stubs. I kinda doubted my skills in getting the axle stubs welded in straight to each other in the beam and straight to the trailer, I'm sure they are welded in a jig at the factory. I was just going to do a bunch of measuring and marking on the trailer to get the full suspension straight. I figured I'd have better luck welding in the full suspension and axles and then end up with a trailer that maybe only dogtracked a little.
We were trying to get rid of the small tires and the small brakes also. He needed a full brake job and was having difficulty finding parts for his.
His trailer was just too far gone to invest all the time fixing it. The rear of the trailer was all broken out, and the neck had welds busted out had been horribly patched in the past, and some serious rust issues also.
That said, I think the switch to 22.5 would be much nicer for you than the old 15's. Its getting harder to find tubes and flaps, and tires. I just think if I was going to all that work, I'd just as soon have air ride (and I think it would be easier) , but I don't know what you have for axles/ suspension laying around. My lowboy runs 255 22.5's. But I'm tri axle so I don't get terribly high axle loads. My heavier crane I haul is only 70,000 or so, so I only have 45,000 or so gross on the rear tri axle.
I guess for me- the trailer better be cheap (under $10,000), and you need to be happy with a ground bearing trailer. If its much over $10,000, I think you could buy what you want ( used) cheaper than you can rebuild that one.