Basically I'm after a higher 5th gear ratio and possibly Lower 1st gear ratio.
I decided to start with the KLR650 and try and put this issue to rest once and for all with a definitive answer.
So far, it's a definite maybe
I started this by using a 90-04 KX500 Transmission and a 87-95 KLR650 Transmission.

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The first step I took was to measure the center line distance between both input and output shafts for the KX500 and the KLR650.
They were only .01MM (0.0004") difference which was probably due to my poor measuring so were good to go on that.

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The next thing I did was count the teeth on each gear.
As you can see, there is no 5th gear overdrive, only a change of first and second
K5 90-04 KLR 87-95 KLX650C KLX650R
Input shaft 16T 15T 15T 15T
Input 2nd 18T 17T 17T 17T
Input 3rd 22T 22T 22T 22T
Input 4th 22T 22T 22T 22T
Input 5th 24T 24T 24T 24T
Output 1st 32T 34T 34T 34T
Output 2nd 26T 26T 26T 26T
Output 3rd 26T 26T 26T 26T
Output 4th 21T 21T 21T 21T
Output 5th 19T 19T 19T 19T
Here's the deal, the KLR650 gears do slip right on the KX500 shafts.
The only real difference was the dog ears on the gears.
The KX500 uses squared off ears whereas the KLR uses rounded ears.

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No problem you could use all the gears from the KLR650 except for first.
I'll get some measurements tomorrow.
I will also install all the KLR gears on the K5 shafts and then attempt to put them into a set of K5 cases and see what happens...
Ok, here's the answer you have all been waiting for.
NO WAY!!!!!
The output 1st on the KLR is larger then the output 1st on the K5 by 2.5mm.
That would mean that you would have to use the KLR shaft which won't work

Sure you could use some of the other gears but you won't gain anything.
Not all is lost.
We have some info on another WRK5 transmission and will be posting our findings as soon as we have more definite information.
If bikes are for kids I'll never grow up.