I looked at the manual on-line: they didn't seem to want me to download it without paying money. I found it at:
http://www.scribd.com/doc/26622228/Lawn-Boy-Service-Manual-1950-88-Complete

Magneto screws normally have a little slack in the mounting holes, and the amount of adjustment required is quite small. I thought you might just have rust and garbage in the holes preventing you from adjusting the clearance. The manual has detailed instructions for making the adjustment.

I was surprised both by the complexity of the engine and the detail in the manual. It appears to be an exceptional 2 stroke, but maintenance may be a problem without a supply of spare parts. You may also need to mount a small seat on the mower's push-handle to accommodate a riding mechanic.

Glad to hear your mother is still active - it may help keep her healthy. Hope she continues to drive safely, and enjoy it.