Don't do anything until more people post with their own advise. My advise is based on quick fix between races type of work.
If you want to do it properly pull the head off, but if you know the head is good you can do it without taking off the head.
Most valve stem seals are crap and will not last very long. Todd Miller in the USA sells Viton valve stem seals and they are very good quality and last forever.
You will need a tool to remove and replace the valve spring. You will probably have to make something.
The hard part, you have to hold the valve up while you take the valve spring off.
Two different ways...
1) Get an adaptor that will screw into the spark-plug hole and hook the adaptor up to your air compressor and use air to hold the valve up.
2) Take out the spark plug and get a length of thin rope, feed the rope into the cylinder till you can't push anymore and then turn the crank to push the rope up to hold the valves.
To remove the valve springs, get a socket (about 3/4-19mm) and sit it over the spring retainer and give the socket a hit with a hammer, this will loosen the collets (they are locked in place by the taper between them and the retainer) If you give the retainer a few hits you can get the collets to jump out.
MAKE SURE THAT THEY DO NOT FALL DOWN THE PUSHROD HOLES (or anywhere else!)
Once you remove the collets the retainer and spring can be removed.
Fit the new valve stem seals and refit the springs.
Do one spring at a time. Dont pull them all off at once.