I did this on a 100 spreader last year. The rim itself was rotted beyond salvage and the center was solid. The rim was actually not welded to the center. It was riveted in place. I ground away the rivets and the two parts separated easily. I then purchased a new rim and clamped them together, marked and drilled the holes, and riveted the new rim on the old center. After painting it looked almost factory with the rivet heads being slightly different from the original. The rivets used are larger than the average hand riveter can handle. I used a pneumatic rivet tool. Find a place that sells commercial size rivets and ask them to sell you just a a few. I told the salesman what I was doing and he actually gave me the rivets AND loaned me the tool to install them at no charge.
(Or...take the low road and weld the new rim on.)