The 5511 has the same final drives as the later models.
The engine of a 6718 will also fit as it is the same basic block.
the engine of a 7711 turbo will even fit.
The new 130 hp Forterra engines will not fit, to fit the 110 hp version in the Proxima Plus, they had to change the casting because the UR3 engine has the starter engine on the left side.
The 5511 will weigh little, but take large engines. I know of a guy pulling with a 6711 fitted with a 7711 engine, putting out about 120 hp.