If its the same as most other east/west engine setups the engine and pump need to be pulled. Its not as big of a task as it seems, a few hoses and wires need to be disconnected then you can remove it as one unit. It will still take a few hours to do I'm afraid and if you haven't done it before I'd work on the best part of a day WITH a helper.
Hopefully someone else can give you more info, i have never worked on a 773, but i am in the process of putting a 751 back together, it has the fuel tank under the engine,. My dad and I put the engine and pump in Saturday, its a 2 person job if you don't have an installation jig.