it's almost certainly a 820 series Peerles/Tecumseh. The ID place is a tine aluminium tag located near the brake assembly, in a recess. Not easy to spot.
If you get the numbers, I may be able to help with a replacement. Need to know the number of forward gears - I assume 5?
Used ones do come up on ebay - going rate seems to be about £60 - about 15% the cost of a new one at full retail price.
phil_saunders(a)bigfoot.com
Phil Saunders, September 2009