You can check the seals without all the special stihl parts. You gotta block off the exh. port by sandwiching a piece of innertube and tin can lid cut to fit between the studs, using the muffler to clamp it. Rubber side to cyl. Do the same for carb., or use the gask. for a template. It sounds involved, but you can cut the tin with scissors, or you can even use something like an old credit card for backing.
Then pressurize with a leakdown tester in plug hole and check for leakage.
No leakdown tester? Just use an easy touch on a rubber tipped blow gun.
Ideally, ya wanna check with a vacuum source also ( any of yer buds got a mighty -vac ) but if ya ain't got one......
If you decide to replace the seals, be very carefull that ya don't booger up the shaft/s, and check for radial play as well as end-play.
And the seals absolutely are replaced from the outside.
It ain't rocket science!
'Course if yer house has more wheels than yer truck...