I just happened on the website of ACtronics, a UK firm that remanufactures automotive ECUs, including the Marelli 5AM series. The company offers units for Grisos and other CARCs as well as Centauros. They maintain a stock of ECUs and do an exchange service. They should be cheaper than buying new.
Besides being a potential source of replacement ECUs, the website has an interesting discussion of failure modes. They say the main problem for Marelli 5__ series ECUs is in the ignition firing circuit, where a failure of an ignition coil is said to lead to failure of the ECU. They recommend replacing ignition coils to save the ECU in this circumstance. This could be something to think about for us.
I searched but didn't find any previous mention of this company on the forum. Hope it will be of interest: actronics.co.uk .
Moto