If the starter spins and doesn't engage anything, then you have one of two problems.
1st, the sprag clutch is slipping. Other fellows have had this happen. Make sure the battery is really charged up then try it again. It seems to slip when the battery is low on power. 12.7 is not that high for a charged battery. If it continues to slip, you might have to replace the sprag which is situated behind the left hand side engine case. It is part of the starter gear train. I think Grezgorz had to replace his some time ago.
The 2nd problem is a cracked case where the started idler gear rides. This is a major problem and could require the engine case to be replaced unless you have some machining skills.
Try the battery before pulling the side cover off.
