2001 Honda Rancher 2wd need advice on frame replacement

My dumb kid neighbor was given a 2001 Rancher 2wd by his grandpa. He loved to ride it on the tidal flats (salt water and mud) but was too lazy to wash it, so the frame has rusted out. They had the engine rebuilt a few years ago. It still runs but he and his dad are clueless about how to fix anything. His research involved asking the dealer about a frame. The honda dealer said $1600, so his dad bought him another new for wheeler instead (!)

I am confident that I can pick the atv up fairly cheap, I’m thinking around $200.

I just need to know what years and models frame will work so I can be on the look out for either a complete gag needing an engine, or pick up a frame from an atv salvage yard.