I've done that with a Guzzi but never with a K...
For the Guzzi, I connected up the battery to the starter solenoid as per normal, then wired in an MO (momentary operation) switch that pretended to be the starter relay. For a compression test that's all you need. To do the brick would be similar; something like this:-
1. Dismount the starter relay from the box
2. Using at least 95-amp capable cable, connect the battery positive to pin 30 on the starter relay, then connect pin 87 on the starter relay to the starter motor.
3. Again, using 95-amp cable, connect the standard earthing point on the gearbox (it is still attached to the engine?) to battery negative.
4. Using ordinary wire (11 amp 16/0.2 or similar; this one isn't going to be carrying much current), connect pin 86 on the starter relay to pin 30 on the starter relay (yes, it's a loop across).
5. Using an MO switch (it has to be MO because if you drop it, everything stops) and the ordinary wire, connect one side of the MO switch to pin 85 on the starter relay and the other side of the switch to earth.
Push the switch and the engine will spin - unlike the Guzzi, I don't think it will try to jump off the bench but be careful. Make sure that the battery is fully charged so that you don't damage the starter relay.
Can someone else please cast their eyes over those instructions? I'm pretty sure they're correct but given what Blackthumb is trying to do, confirmation would be nice for him.
One last comment; although you'll probably think it's a PITA, use proper crimps and connectors on the cables and wires. You do want everything to stay secure.