Your local MTD service center (they would sell White, Cub Cadet, and other low end brands) should have all sorts of pulleys. What you would be looking for is a keyed one that is a larger diameter than the one you already have. I'm not sure of the dimensions, as I haven't had an MTD rearend out to look at in a while.
F-250 XL RC/LB 4x2 Power Stroke Diesel
2010 John Deere
Z710A 48" cut
1999 John Deere
1985 John Deere
212 w/38" deck
1983 John Deere
420 w/54" hydraulic front blade
1974 John Deere
140, 54" hydraulic blade, 3pt. hitch, tandem disc, plow, 48" deck
1971 John Deere
112, 38" deck, all original, down for engine rebuild.
1967 IH Cub Cadet
124 w/creeper box, disc, plow
1947 Economy/Power King
Tractor S/N 590, fully restored.
Certified John Deere