When the transmission is removed, it's a good time to clean the Gear Digital Display switch. When it's clean, coat it with silicone grease. They are not waterproof and a lot of riders experience problem when riding under heavy rain conditions. The following picture is the switch. The arrow is pointing at the area needing silicon grease. You don't have to open it, but if you do, work in a clean area as little springs and pins will fly out 50 feet away.

First you should have the following new parts available to reassemble your clutch (see picture of the part list):
1 Hex nut, 6 screws, 6 washers, 1 O ring

You will have to build a support to put under the back of the frame:

You should have the following tools available:
All the basic spanners and Allen keys, a 30mm socket (1" 3/16 works fine), 1 seal pick, 1 centering tool or a cheap Papermate marker, 4 8x90mm screws (this is to support the transmission when reassembling), and a torque wrench.

Here is the dimension of the centering tool if you have the opportunity to have one made:

Before dismantling the clutch, read the following:

Originally the clutch position is marked from the factory.
From experience, I engrave my own marks as over the years the original one may have wore off and it would be difficult to re-balance the clutch.

When the transmission is removed, dismantle the clutch by removing the 6 Allen screws (#7 on the picture) Then remove the clutch plate, pressure plate and diaphragm. Mark their position in relation to each others.

It is time to remove the nut holding the clutch housing using the 30mm socket. To stop the clutch rotating just insert a piece of wood (Hammer handle) in one of the 3 big holes on the housing. (The wood will rest on one of the ribs of the engine housing).

When the nut is removed, you will have to dig out the O ring #11. Since it has to be replaced, you can cut it!

The clutch housing can be pulled off the shaft.
WARNING: The stop disk # 10 may get stock on the housing or on the shaft. Its is only 1mm thick  and stick with the oil. Don't lose it!

The output shaft seal is now visible. (This picture is from a spare K75 engine and the clutch bell has been removed)

Pop out the seal and before inserting the new one check The brass bushing on the output shaft. This is where the clutch push rod rides.
I have not found any part # for this bushing, and if it shows too much wear, it may have to be remanufactured by your local machine shop.

I am using a metal plate to gradually tap the seal in place. ( Do not beat on the seal with your hammer.) When it start to get into position , check that it's going straight.
IMPORTANT: put your old hex nut on the shaft to avoid damaging the thread!
Here is the simple solution that I use. My plate is drilled to go over the 30mm socket (snug fit). I put the socket on the nut and it will hold the plate centred with the shaft. I don't have to hold the plates with my hands (and beat my fingers).

THE SEAL SHOULD NOT BE FLUSH WITH THE HOUSING! IT SHOULD BE LEFT 1 MM OUT.

Insert the clutch housing on the shaft, install the new O ring and thigh the nut to 140 ft/lbs (foot per pound) or 140 Nm (Newton per meter)

Re-assemble the clutch being careful to line up the marks that you did before removing it.

Additional information:

Check this thread about the proper installation of the main shaft seal

Check this thread for tips on re-installing the gear box