Available space.

It is obvious you'll have plenty of space to do anything you want inside the fuselage, the available space is really big and without any doubt you can install here at least 4 liters of fuel.
There is a fuel tank included, but for turbines this one is good for the bin (too small volume : 550cc). The manual speaks about 150-180 two stroke engine with a pusher prop .
I placed a Futaba R606FS FASST receiver on the tray to give an idea of dimensions.
Just been checking some options with some available tanks. Here two Sullivan BT-32 (32 ounce fuel tanks / about 960cc each) placed horizontally and vertically. No problem.
Even two tanks of 2 liters must be possible to do, by moving one bulkhead.