Look on the engine for a "designation flat" that has been ground smooth and stamped with a Toyota designation ID for the engine.
In this case, the designation might be 4P.
I have access to a 3FG manual which gives the timing spec as 8 degrees BTDC. Of course, 8 BTDC is for a gasoline fuel configuration, so add about 4 to 6 degrees to that for LPG (12 to 14 BTDC).
I'm not sure you understand the reasoning behind the need to advance the spark point of an engine, but the need for advancing it becomes even more important at higher RPM than at idle, and that is why there are vacuum and centrifugal advance mechanisms in the distributor.