The 1.3 multijet is a super little engine when working properly, and will work properly for a very long time if serviced on time, and with good oil. I'm not a lover of diesels, but this one I could live with.
As far as I'm aware, the Doblo alternative engine was the 1.4 petrol, initially in 8v, then later in 16v.
The engine ECU (computer) communicates with the body ECU. Connecting a petrol ECU to the original body ECU is likely to fail. Therefore, a swap would need both ECUs, which then means all the locks and ignition switch to avoid immobiliser issues. Simplest way would be a wrecked petrol model as a donor, then issues of disposal of the wreck.
If your engine is broken beyond repair, replacements are usually plentiful, being used in Fiats, Vauxhalls, and some Fords. I think the Bipper and Nemo also used this.
If you just don't like it, swapping the whole vehicle is probably easier, but petrol Doblos are scarce as vans, and most cars are disabled conversions, bringing us back to the wreck donor I suppose.
Good luck.
A V8 Mustang lump is probably too difficult, unless you mount it in the rear.