The Sealions, latterly built by Toughs, all had Perkins HT 6354's, I believe, and were all 40 feet long.
I suspect that with Fords it has been re-engined from a petrol driven original, so check who did the conversion, were the engines new or recon' and were the outdrives replaced?
It will have been built with petrols and now got a pair of transit engines.
I have driven one or two extensively when they were new, I thought they were pretty awful in most respects and represent the worst perios in Sealine history.
They do however offer good accommodation for 30ft.
At a guess, they may be the Lancing Maring diesel versions of the base transit engines.
if so it would depend on which varient. The 140hp version would be fine in a 305, but the lower hp versions would struggle.
edit.. damn, lakesailored by BB
check carefully the quality of the installation, if it's a decent installation , then they are good engines.
Glass filter are a problem on the rivers due to the Boat Safety Scheme, but not aware of any problems with RCD. People like ASAP spares are selling galss filters with a flame guard for river use.