If they are the tapered seat style then you can sometimes get them to seal by giving them a very light tap on the handle end of the spool.
The original version of drain cock is available on ebay..
www.ebay.com/itm/291580174809?hash=item4...tkp%3ABk9SR7K5oNyDYQ

Lapping them in like you tried sometimes works but it can cause the tapered spool to sit too far in the valve body.

Try Florin Tractors in Sacramento, they have the brass peacocks under the same part number

Regards

Here ya go,
www.mcmaster.com/petcock-valves/body-material~brass/

Thanks for your input guys, I also messaged Toby Nelson on his YT channel and he came back and told me he used these ones on 5J1113: flywheel-supply.com/tee-handle-hex-shoulder/#/

Today I made a puller to extract the brass ring retaining the Thermostat and managed to get it pulled with only a little deformation. However on testing what I presume is the original thermostat, I find that it doesn't even attempt to open even in boiling water. 
 
My parts book gives it a part number of 4F7059 which doesn't bring anything up on CAT's online parts checker. However doing some searching it looks like if crosses to part number 5H0597 or 6F9467 which has a cost of $240.16NZD!! Ouch!
This thread here gives some useful hints: www.acmoc.org/bb/for-sale-wanted-d6/2384...regulator-thermostat except it relates to a D4 7U
Can anyone confirm the Gates 33388 (180degree) will fit (at a fraction of the cost)? I.e. like this: www.stockwiseauto.com/gates-33388-engine...d=2&gsID=pwf865646g2

I wonder if the one you've pulled out would be able to be freed up?

Haus wrote:

I wonder if the one you've pulled out would be able to be freed up?

I wondered that, and managed to compress the spring and remove the valve and the two straps. I tested like that as well, with no luck, but I'm not sure how to dismantle it further?

I managed to pull the piston, clean it and lubricate it as described in the manual, and now the thermostat opens, but I don't think its enough, can anyone tell me from experience? That's at over 90 degrees Celsius (should be fully open at 84 degrees Celsius, or 180 degrees Fahrenheit)

 

Keep heating and cooling it, it will get back in shape.

Rome wrote:

Keep heating and cooling it, it will get back in shape.

How many times Rome? I did it probably 10 times this morning an its no better.