I have a 24 hp 48" deck Craftsman lawn tractor, model 917.275702, just over
two years old.

I have always (until this past month) started it at least weekly to maintain
the charge. I let that go for about the last month, and now the engine
won't turn over. If I switch the headlight switch on, there are no lights.

My guess (but not certain) is the battery is now undercharged.

If that's the problem, I need to recharge it or replace it. The manual says
don't jump start the 12V battery unless your source is 12V. Dumb question -
is a regular car battery 12V? Can I jump the tractor battery with a car
battery?
Should I buy a charger? The manual instructs to recharge at 6 to 10 Amps
for 1 hour.