Havn't any experiece of the Sole. The only reason for my Lombardini (apart from twin and fresh water cooled) was that my 'yard' offered a good deal and they were the registered dealers. To some extent you have to go on what the local guys know about if you are not going to fit it yourself.
Without knowing much about it, now that the tax will be equal for petrol and diesel, a petrol engine, if you can ignore the explosive side, seems a reasonable bet ... Stuart Turner still do one? Smooth and quiet I think. I don't envy your choice problem!! Chris.