I have had my original Mk1 Carrier carb witht he governor rebuilt by Treadwell Carbs in NY. Cost was $80 USD. Took a while as they do race car carbs, but they rebuilt it. and ran it in on a v8 befo0re shipping.
http://www.carbsandkits.com/tc/treadwell.html
They know what a bren carrier is......
Cheers,
Jim