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Several years ago my father converted his 115C from 6 volt to 12 volt. I know that there will be many who will say that he shouldn't have done that, but he did.

My question is, will the tune up specs remain the same even though it is now a 12 volt system?

The engine, distributor and advance curve would all have stayed the same so I would think the only thing that might possibly be changed would be the spark plug gap. A 12v coil producing maybe a hotter spark might have tempted your dad to use a wider plug gap more in line with current specs at the time of the conversion but that would not be a given or even necessary.

For 12v, the spark plug gap is typically .032" and, if negative ground, that's what I would it to. However, my experience with the 10mm plugs is decades old so someone else might have newer information that applies to these small plugs, which I am presuming yours has.