(ET) shunt for Alltrax E15 retrofit - what specs, or how to substitute?

[<Joel Parks nokia com>]

My E15 (that I picked up off EBay last month with mower and snowblower) 
wouldn't move under its own power.  Opening up the control box, I found a 
rat's nest.  Actually the previous owner said they were chipmunks, but in 
any case it was a mess.  Lots of wires were chewed and corroded and I 
didn't know where to start.  Since it is a HA (relatively late-model), 
most of the diagrams I've been able to download didn't relate to my 
circuit with 5 contactors and different control logic.  Somebody's website 
I saw mentioned a conversion using a Curtis 1204 or some such controller, 
and I decided new technology was the way to go.  Luckily (since I'm not an 
electrical engineer) before I ordered one of those I found this list and 
discovered the Alltrax controller.  I ordered it last week and started the 
installation.

I'm nearly done but I've hit a couple of snags.  Steve Richardson at 
Alltrax answered my questions about the external resistor (1 W 10 Ohm) and 
diode (3A 100V) shown on their schematic - it turns out I need to obtain 
those myself.  

But the big problem is that my original circuit didn't have a shunt, at 
least not one I recognized as such.  And if it was just a length of wire 
like Steve said he saw on a E20, then I have no idea which wire it was.  
Evparts.com lists a pretty good selection of shunts 
(http://www.evparts.com/shopping/?id=943) under the Renewable section, but 
I don't know which one to get to match the ammeter in the E15. 

Also, I re-used one of the leftover open-frame relays to implement 
key-switch and seat-switch interruption of the PTO contactor, a feature 
originally present in my machine and not shown on the Alltrax schematic.  
Since I have an almost 3-year old around, I thought the safety feature was 
worth an extra 1/2 hour of crimping and locating the right 
interconnections to use.

Steve suggested that the thermal cut-out switch mounted on the drive motor 
could be rewired in series with the key-switch and seat-switch, rather 
than in series with the M- lead, this would serve the same effect and 
reduce the current carried by the thermal cut-out and thus any arcing if 
the cut-out opens.  Using a 18-gauge wire would simplify rewiring for 
anyone who's removed the chassis wiring.  I guess if I do this I'll add an 
override switch so I can debug which switch is failing (key, seat, or 
thermal cut-out).

Questions:
Should I get an off-the-shelf shunt for this meter?  Or if I'm better off 
solving my problem the old-fashioned way (fabricating my own shunt), does 
anyone know the full-scale millivolt input current for the OEM ammeter?

Also, what wire gauges are used for the battery interconnects and for the 
motor power and PTO connections?