Rod has already very kindly done that and his link will provide you with all the info you need to get your "rough value".

The problem is, the BSFC of each engine differs.
For example, pop a set of 803s in a CVH and see if it will do 350bhp on them like a YB does.
It wont, as it has a lower BSFc than a YB which, for a tractor, is surprisingly efficient, even by quite modern standards.

The only way to get an accurate BSFC is a dyno cell with an accurate fuel delivery monitor and work out how much horsepower your making from any given input of fuel. This drops as engine efficiency increases.