The used M18 would be quick and easy if It's in good shape! Those old opposed twins are hard to beat for low end lugging power and torque
If that doesn't work out the Vangard kits from Small engine warehouse are hard to beat... drop in ready!
Like Tim (olds) said.. take care of a few other things while you are in there and you will have a like new tractor! 
__________________
Jay
40 years of Using and playing with IH Cub Cadets!
Proud owner of the following:
Cub Farmall, Super A Farmall, Original, (2)70's, 72, 100, 102, 123, 105, 125, 127, 108, 128, 1450, (3)782's, Yellow 982, 1782, "Sam's" 2182, M Farmall and a #7 trailer
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