How do I get 12 volt power from my 3.5 horse briggs. I want to power up a head and tail light.

You can't do this unless you have a engine that has an internal or external altenator. It does not sound like yours came that way. You can add a 12 volt battery but it would need periodic recharging which will not come from the negine.

Anyway of adding an alternator? Or any other options? Magneto?

I'll give you this link: I suggest you contact them. Electric Starter Kits for Briggs and Stratton Engines.

None that I know of for such a small engine, sorry.