I am sure you can put the chain on by yourself, with some time. Is a come-along part of your tool collection? Husband (not home to ask) has repaired our spreader chain with new link additions, new slats, and the come-along was always part of his tools used. He used it to pull the unconnected parts together, giving him slack to lift and connect the chains without tension on the ends. Same for replacing slats, you put the hooks PAST the bad slat, remove it, then put on new slat. You then connect the slack chain ends, then remove (SLOWLY) the tension on the come-along, watching that chains lay flat as weight is applied. Check, that you got the same number of links between each slat, so gears pull chains evenly in work…
Your chain will sag a bit under the spreader floor, to have room for slack when you need to work on the chain links or slats. Usually the teeth of gears (at both ends of spreader) do turn freely if ground drive is off. This allows chain slack to be taken up or let out when work is complete.
He learned by doing the fixing, I helped, handing him tools, working the come-along as he figured things out. Now he fixes spreader chain by himself. One of my nicest Christmas gifts was a full set of slats/bars he made for our new-but-used, elderly spreader! The old bars were BAD, along with many of the chain links. Sure was nice with the replacements on!
All our spreaders had chains ABOVE the axle as they revolved. So make sure chain and bars are laid out that way before trying to connect the two ends together. Husband had to help a friend that tried to run chain around the axle, could not figure why chain was too short. Friend was really tired after wrestling with repairs all day.
I am sure you CAN get the new one on, get it working properly. If possible have a 2nd person to help hold things, fetch tools, second set of eyes looking at things with you,. It just is easier than being frustrated alone. Allow plenty of time to complete the job, so you don’t feel pressured or hurried, which can lead to mistakes or injuries when tired and rushing. Be Safe!