I'm also planing to build a small manure spreader something to pull behind a lawn tractor.
I got a big PTO driven spreader, but it's to big to get in a few places. I also have two ground driven spreader that needs new wood. It might be cheaper to get a old spreader and rebuild it. Most needs new wood.
I'm planing to used a old grain chain elevator and using it for a spreader. I got hold of three of them. And like to try to make a few of them each will be different. First I will use the elevator troth and chain with a 5hp gas engine. 2nd one will be wider then the first. I may try to run them on a pto, ground driven, or a gas engine. I also like to try to make a side beater. So I can drive along a tree line and throw manure off to the side or use it for mixing compost.