I have an older Sears Craftsman lawn tractor with a 180 opposed twin
cylinder Briggs and Stratton Engine. My son was cutting the lawn with
it the other day and it gradually stalled out on him. He started it
up again, but it quickly stalled out again. Now the engine will barely
crank and will not start.

I checked the battery with a multi-meter and it tested at 12 volts. I
tried jumping it with a portable power source and this did not help. I
tried a new on/off switch and this did not help. I removed the cover
and I can turn the flywheel by hand without difficulty. The oil level
appears to be ok.

Any suggestions of what I should do next. I don't have much
experience with engine repair, but due to the tractor's age I don't
think it would be worth taking to a repair shop. So I willing to take
a shot at home repair.

From what I've read I am thinking the starter engine might be the
problem. One internet source suggested jumping the starter directly
from the battery. I am willing to try this, but it appears on my mower
that I need to first need to remove the flywheel to get to the starter
and this looks like a major undertaking, especially given the age of
the engine.
Is the starter where I should start or are there other things I
should or could check first? One question I have is whether a problem
in the starter would have caused the engine to stall out after it was
already going?

Any other suggestions on how to troubleshoot this issue? I do have a
basic Briggs and Stratton manual for the engine on a CD-Rom, but it
does not contain alot of trouble shooting info (at least that was
helpful to me) on the starter/electrical system. It is helpful,
however, on assembly and disassembly issues.