I have suggested, elswhere on this forum, that they put in an inshore minesweeper.
This, without the gun and sweep, was used as a training vessel for the RNXS and also some were converted into survey vessels and one into a torpedo recovery vessel.
This last would be ideal to use as a dive mothership as it could launch and recover smaller dive RIBs.
I have both photographs and a set of lines if they want to include it.
The dimensions are 120 tons, 105 feet between perpendiculars, two Paxman 12 cylinder marine high-speed diesels.
Also, for a different propulsion method, the "Brave" class partol vessels had marinised gas turbines also the RAF Air Sea Rescue boats had three marinised Merlin aircraft engines. They are fun as you can use the outers for manoevering, the middle one for cruising and all three for a high speed run.
As to the security aspects, anything in Janes Fighting Ships is in the public domain.