Hi Marc.
Welcome to KX Riders.
There are tons of good guys on here with a wealth of information.
I know you will enjoy it.
If your kick starter is slipping, it may be something as simple as a weak spring.
However, it may be the gears on the kick shaft are slipping.
Sounds bad but it really isn't.
You don't have to split the cases to get to it.
All you need to do is remove the clutch cover and it becomes obvious

If you don't have a good service manual you might want to invest in one.
Enjoy!