If it's a 2004 Disco 2, instead of a 2004 Disco 3 (Wikipedia has both of them available as 2004's) then you could import a JDM/euro front clip with the Td5 diesel or possibly a whole vehicle from 1998/1999 under the Canadian 15 year exemptions
A comparable beast would be a diesel Toyota Land Cruiser, they're going for $20k in good shape, and half of that for a front clip. A diesel Toyota 4runner is $10k, a clip again about half that.
Tough call. Amortize it out over 5 years, would you save $2k a year in fuel, plus the cost of your time to figure out the conversion? The diesel should get 25 - 40% better fuel economy depending if it's city or gentle highway cruising.