John Deere L110 lawn tractor and the mower blades will not stop when the
Mechanical PTO is disengaged. I think this is a problem with the safety interlock circuit.
Where is it, and can I replace it myself if I order the part?

Hi,
I would severely doubt it is safety equipment. The switch is, however, under the dash and can kind of be seen in the battery photo toward the upper right (where the wires go.)
OMGX21647_K3
And the parts breakdown is at:
John Deere - Parts Catalog
And check out the deck belt picture and the spring #11 (part #GX10120) since it provides the pull-off pressure for the deck engagement cable.
Peace,
Clarke