After a bit of consideration I decided there was no option but to remove the original prop tube and rudder. The prop tube came out easily enough with a drift and a few light hammer taps. Thank goodness it wasn't glued in all that well. A modern 6mm prop shaft should fit nicely in the 1/4" hole.

The rudder was more of a pain. It came away from its glue easily, but wouldn't come out of the hole. Plus I knew I'd still need access somehow for a servo. So, reluctantly, I removed the seats and rear panel. Again, thankfully the glue was a bit crap and cracked free fairly easily to reveal a nice empty space and the top of the rudder.

Next decision: fit the servo there, but have to somehow make it accessible in case it fails? Or run a rod linkage towards the bow and fit the servo where it's much more accessible, sealing the linkage behind a panel forever...