You know, I hope, that they don't sell the diesel in the states, but the principle should be the same. Make sure your plugs and wires are up to par. Chrysler puts crap into the OEM engine and your about at the point where plugs could be due for an upgrade. The other two things to look into are exhaust and intake. Neither will give you a kick in the pants kinds of extra HP your looking for but might be enough to help. Look for a less restrictive muffler and an intake system that will draw more air into the throttle body.
Since your in the UK, have you tried this site for cruiser parts?
http://www.ptcruzin.co.uk/
...and, do you know about the UK Cruiser forums?
http://ukcruisers.net/phpBB2/index.p...0c06ab868bc2c7