G'day isie.
I am not 100 % sure what you are saying ??
The thrust washers have fallen out,if so this is caused by the operator driving the tractor with his OR HER foot resting constantly on the clutch pedal and eventually wearing the crankshaft thrust bearing down thin enough that it falls out and when the rear thrust washer falls out this will allow the front thrust washer to fall out then the crankshaft will contact the front main bearing cap, normally you would feel the clutch pedal loose and it sometimes shudders as the crankshaft GRINDS the front bearing cap.
To repair the problem .
Providing the crankshaft,main bearing and engine block thrust surfaces are serviceable it could be as simple as measuring the TOTAL FREE PLAY of the crankshaft and installing new oversize thrust washers.
If the engine block thrust surface is damaged it can be repaired by Brazing the damaged surface and re machining back to the standard dimension ,this I would say is better done by a machine shop with the specialist knowledge and equipment to complete the task.
New crankshaft and connecting rod bearings are supplied with the journal size stamped on the Back of each shell Be it STD (standard) or + 10+ 20 + 30 etc, check the size of the bearings that are removed and I would also measure the Crankshaft Journals to be sure, mark or make sure the caps are refitted in the same journal and the correct way as they were removed.
If as you say the engine will be removed from the tractor ,Which in my opinion would be the way to go, You would be wise to check the condition of the clutch , Replace the Front and Rear Crankshaft seals, It may also be a good time to have a look at the engine lube oil pump.
That,s all I can offer.
as always .
Happy Days.
Hutch.