Tractor question... sorry, tractor is also a kind of car

Craftsman model 917275180 Tactor.

What's the difference between battery type U1 and U1R? Can I use U1 instead of U1R? The original parts from Sears is too expensive, so I am trying to substitute a generic brand.

Also, I just did tune-up myself last week (did it every year): oil change, oil filter, air filter, spark plug. It was running great after the tune-up, today I couldn't start the engine... I guess it's the battery. What else could be the problem?