Bugger!
if the thread on the crank is not too badly damaged you may be able to clean it up enough to get it started with a ground down hacksaw blade (remove the pulley for better access, it should just slide off as it is on a key and not on a taper)
grind the teeth down (to a point) on the hacksaw blade to match the thread pitch of the thread, now lightly saw down into the valley of the thread working slowly around the crank until at least 4 threads on the end are clean enough to start the nut.
i am assuming you purchased a new nut? as if the old one was stripped you cannot just re-thread it! or it will be oversize and it will not take any strain before stripping again.
DO NOT use the tractor without the nut fixed and done up tight![/] (i think about 100ftlb) or the pulley will come loose and flog out the keyway in the pulley and the crank, and then you will need a new crank
Probably best not to drive the hydraulic pump of the nut!!!
Drill and tap some M8 or M10 (6 is ideal) holes in the pulley and make a suitable drive flange that you can bolt to the front pulley (using Grade 8.8 or better bolts) that will take the load of driving the pump .......... ie weld a coupling sprocket to big (say ~60mm Dia) washer, weld on 6 spider legs from 25x5mm steel strapping that bolt to the front of the pulley. (you may need some pipe spacers to get a good fit)
hopefully you can make it so the total length of the coupling still aligns with the pump (ie if the back of driving flange is touching the front pulley nut and the coupling sprocket has 5mm machined off the back it should all fit)
Tip..... make the flange and bolt to the pulley first then find the true center with the engine running (or cranking) so when you weld on the coupling sprocket it will be centered
(do not weld while bolted to the crank, index one spider leg and it's bolt hole on the pulley so you always bolt it back in the same position)