Just something to think about: (food for thought $ wise also_
15 year old mower and still has same belts, pulleys, deck spindles, etc.
If this is the only mower you have and grass is growing while you are working on the old rig you might want to think about limping it until mowing season is over. It will at least need all belts replaced and YOU WILL FIND OTHER THINGS THAT need attention.
You might even find you need $$$ ($) to find a replacement mower or a hay baler.
More attention can be given to re-furbishing a OLD mowing rig when it's not really needed and the weather is cooler.