Photos are at http://www.classicmotorcycleforum.com/index.php?topic=4744
A bit of trial and error called for, as even on a simple bike there are many things that can sieze. On your bike the likely candidates are motor, generator, chains, gearbox, brakes (even if they're not connected) or wheel bearings. For what it's worth, I'd start like this:
Put the bike on the rear stand.
Remove the spark plug.
Remove the primary drive cover. (Looks like a tin affair, so undo any nuts/screws you can find and see if the outer cover slips off.)
Make a cup of tea and tell your friends to go home. Don't hurry.
Are any of the chains dead tight on both runs? They should all have some play, and you can use this to see if any of the rotating parts are tight. It should be relatively easy to rotate all the different parts you now have access to, so with some gentle rotating you should be able to identify anything that is "tight". If a chain it trying to rotate a tight component, the run of chain that is pulling on the sprocket will be very tight.
If everything seems OK in neutral, select the three gears and rotate the back wheel to see if there is a problem in the gearbox. You can take the bike off the stand an wheel it around in gear to see how it feels.
With a bit of luck, you might find the problem component.