dude your timing should never go out on those. you sure you're checking it right?
also have you tried a new belt yet> it makes worlds of difference on those motors for real real real. like a shitty belt will feel like everything else is wrong
OK back to adjusting timing, yes it's possible. sortof. but not simply. So you basically have to change where the timing pulse occurs with respect to the crankshaft. You can get the stator cover and stator open and pull the stator out and yea either gauge out the mounting holes to allow you to rotate the stator somewhat, or just drill new holes to mount the stator in a different position (x degrees off from where it was to match it up at where your timing should be)
The other option is to take the stator out and take the magneto off, remover the woodruff key and rotate the magneto accordingly and press it back on, but i've never taken the magneto off on those motors and it may not really be easy. I think they use the same flywheel puller as sprees and express SR. not certain tho. that may be the easier option especially since it may have sheared the woodruff key anyway to knock it out of timing. just clean the taper well - i use valve grinding compound between the flywheel and cran ktaper and spin that a while while pressing in to clean and match the tapers, then clean it off and press it on appropriately where the timing lines up rightly. it should also be easier to see when you've got it right. little less durable than the first method though, a little easier to foil.