On the Tomos I have worked on... the plate under the flywheel that the ignition parts are mounted to... has slots for the screws.
So you can rotate the plate a little to change the timing.
But if you want to be accurate... you have to find out 'where' it is now.
You need the flywheel to have 2 marks on it... T for TDC... and F for fire.
And the engine to have 1 mark on it.. (for the other marks to line up to)
The ones I've seen only have one on the flywheel... and the engine has none.
So you have to make your own marks... and use a bent coathanger attached to the motor to line them up to... THEN you really have to use a strobe timing light to see where its firing at.
You aren't going to gain a whole lot by changing the timing.
Want to try it 'cheap and easy' ??
Remove the flywheel and mark the screw positions on that plate with a scriber (or something permanent)... then simply rotate the plate a little and tighten them back down and go ride it.
You can always move it back to the marks you just made.