Drive belt on Craftsman tractor will not stay on

My brother and I replaced my tractors engine with a newer engine, now my drive belt will not stay on after I stop and move to a new gear. I put the belt back on a number of times and it kept popping off. When I was checking the composite and idler pulleys they seemed like they were not aligned with each other. One felt like it was higher by ~1\4 inch. I replaced the drive belt because when this first happened I thought the belt had broken, it was the original belt and my tractor is almost 20 years old. After changing the engine I almost got the whole yard cut when the tractor stopped, I really think the mower God’s have a beef with me or Mother Nature does not want a hair cut. My question is are the pulleys supposed to be lined up and is there a way to adjust them?
Thank you in advance
My tractor is a LT1000, but I forget what the new engine is.

he trans pulleys should all be in alignment.
If you took the pulleys off, check you did not put them on upside down.
They have a top hat bush inside the inner race which spaces the pulley.
Easy thing to do on some mowers .
Other than that, did you put the keepers back around the drive pulley ?

bertsmobile1 said:

he trans pulleys should all be in alignment.
If you took the pulleys off, check you did not put them on upside down.
They have a top hat bush inside the inner race which spaces the pulley.
Easy thing to do on some mowers .
Other than that, did you put the keepers back around the drive pulley ?

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Thank you, I’ll have to check on this Thursday or Friday, I think we did pull off the v-grove pulley and that is the one that is slightly lower. As far as the keepers they did go back on — there were no extra parts left over — we just had to loosen them. Thank you again.