Two axles, hubs, bearings, leaf springs, bolts, U bolts, hardware (backing plates), etc is going to cost more than a good used trailer.
Replacing both axles without airtools, cut off wheel, sawsall, MAP gas torch, and a solid workspace is going to be extremely time consuming and potentially dangerous with the boat still on the trailer (and you working underneath).
Dump that old POS rotten trailer where it belongs . . . and find a decent one for $700 - $1200. V20 can travel on a single axle trailer just fine.
Think about what you are talking about 12 hours of driving . . . just to double check measurements. $500+ for 2 axles, $300 - $500 for hardware, 3 weeks of waiting, 6 hours of driving up to do . . . a solid 6 - 10 hours of work . . . (god forbid you don't have everything to do the job), and another 6 hours return trip.
Where are you located? Maybe someone here has a trailer you can borrow for the trip or can give you a deal on a used one.
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Hopefully . . . this doesn't sound harsh . . . I'm just trying to share my awful experience doing nearly the exact same thing

5 years ago. I was lucky and found 2 new axles at Marine Connection Liquidators for $100/each with hubs, bearings, bearing buddies. It still cost me $260 for new tires and 2 new wheels, Bunks (that I made), hardware . . . etc. I was up to $700 in no time before 12 hours of hard labor.