A head port, Jenvey TBs and an engine balance & rebuild and remap on the standard Caterham SLR exhaust gives about 215BHP. I don't know what your power is at the moment but the cost will be circa £3000 if you get someone like DVA to do it.
I would take that route if I bought a secondhand SLR of your vintage unless you knew the engine had be rebuilt reputably or well cared for just for peace of mind and to get the best out of the car. You can of course fit new cams £300 and bigger valves if required 1444 with DVAs bigger valves gives the BHP DJ quotes.
Personally I am please with my standard but Scholar blocked DVA'd VHPD engine and don't plan any more engine upgrades for the foreseeable future, probably some driver upgrades though 😬
I would get on the phone to DVA tell him what driving experience your after and take it from there
Andy