Probably the easiest and cheapest would be to buy the back half - transmission and rear end from a 4000, 4600 or 4610 and swap it with what you have now. Better yet, buy a whole tractor and part it out.
I bought this 4600 as you see it for $1200.
Sold the tires for $300, complete but stuck engine for $300, front bolster for $100 and think I have the rims and centers sold if the snow ever goes away for $175.
The rest went to my great pal Kenny in one of our infamous horse trades.
He's building a tractor out of it.