That engine was made on 4 January 2001. Here is an illustrated parts list:
http://www.morgantownoutdoorpower.net/pages/OemParts?aribrand=BRG#%2fBriggs_%26_Stratton%2f12F802-1975-E1%2fAir_Cleaner%2c_Exhaust_System%2f12F8021975E1%2f05170004
As I read it, Item 1387 is the governor spring and the Briggs part number is 790849. However this is dependent on your reading of the dirtied-up numbers being correct, the parts list being correct, etc etc. However the 790849 is available from:
http://www.ebay.com.au/itm/Briggs-and-Stratton-Genuine-790849-Air-Vane-Spring-/111059937042?_trksid=p2047675.m1850&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D11%26meid%3D7233637680839496401%26pid%3D100011%26prg%3D1005%26rk%3D1%26sd%3D111043720995%26
http://www.amazon.com/BRIGGS-AND-STRATTON-790849-Improvement/sim/B00524NXN0/2I can't find it in the Outdoorking store, but I'm no expert in searching there. If you send a Private Message to Bruce, he will tell you the correct answers, including whether it is the right part and whether he can supply it.
Just about any US Briggs dealer will have it, but the shipping cost might be horrendous.