The procedure demonstrated in this video will be relevant for the models mentioned and also most other overhead cam small engines.
IMPORTANT: Replacing a valve seal isn't always "the answer" to your problem. If your engine is burning oil it can be a damaged/worn cylinder, worn/broken/dis-tempered piston rings, worn valve guides and so on, you get the point - Oil can't get into the combustion chamber. (Aside from what soon gets wiped away by the oil ring(s) of course)
HOWEVER: Oil can come down through a valve guide, therefore they can technically be the culprit. Traditionally, the seal on the intake valve is the most important. (Think about how a 4 stroke engine works: Suck, Squeeze, Bang, and Blow - it's four different strokes) - The first stroke requires the intake valve to be open so that the engine can "suck" in a fresh mixture of fuel and air. This vacuum can easily suck oil down through the valve guide, if the engines has a lot of vacuum that is. On the exhaust stroke (Blow) the exhaust valve must be open, and this can result in two things. 1) Oil being blown up into head and 2) a valve guide wearing out quicker, therefore resulting in an easier passage for warm oil. - But I won't get into this too much.
BASICALLY - If you want to ONLY change your valve seal(s) without tearing down the engine, messing up your timing etc - Just do it this way.
You will (as described in video) have to remove the seat, gas tank, sparkplug, valve cover and cam holder with rocker arms. This is still easier than removing the head entirely. (Also NOTE that valve problems can also result from the actual seating of the valves, as in: they are leaking around their seat in the combustion chamber area)
The shoelace trick like this is known by several people already - But like I said at the start, I'm offering it to those who are relatively new at servicing their bike. PLEASE take the time to check your valve clearance while you have your valve cover off, that procedure (along with the timing process) can be found in another video of mine on Youtube. .... Thanks for reading/watching.