Ralph,
Sounds like a very interesting project.
While not a prison ship per se, the HMS Enterprise was converted from a fighting ship to a receiving ship that held men kidnapped by press gangs as prisoners until they were forced aboard fighting ships. There are detailed plans on the RMG Collections website and many of them in high resolution on the Wiki Commons site. The internals are all different, Things such as the masts, steering wheel and guns are gone. You can start with the Drydock Enterprise model as it shows some things as a receiving vessel and some as a fighting ship. You can ignore the sailing/fighting ship paraphernalia and follow the RMG plans for a receiving ship to make the changes on your own. As with most things of that era, every hulk was different. There are a lot of contemporary prison hulk drawings on the RMG website which are free in low resolution. Perhaps you can pick one and match it to a similar size ship kit. There are plans of a 50 gun, 74 gun and others that were converted from warships to prison ships.
Allan