Hi Redeye, what went wrong with your Google search was a reading error on the model. That engine is a 12Q902-0135-B1, with a code of 09071462. Date of manufacture was therefore 14 July 2009. Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/29jnuIO.aFzXFix2.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/iotyALJU0HujhJvcv4.pdf

The governor spring for your engine is Item 209 on Page 10 of the IPL. The version that suits your Type Number is on Page 16, and is Part Number 796484. Please check this yourself to verify it. You can also see a picture of the part, if you google the number.

I suggest you contact the Online Store. The part number does not seem to be listed, but the 850 is a relatively new type, and I think they tend to list new items as they get enquiries about them. I would be very surprised if they cannot supply the part to you.