On the advice of another converter, I've downloaded the service manual for the N13. Looking at the heater section, I was puzzled by the description of the heater, since it lacked any information about the heater core.
After I'd packed away this morning's tools I went back and took off the glovebox and had a look behind it. Sure enough, the fan unit is next to the door above the passenger footwell. Next to it is the box I'd assumed was the heater core but on closer inspection has no fluid cables protruding through the firewall.
So, I unscrewed the 3 points, detached the electric plug and the unit fell out in my hand. Turns out my version of the N13 has an electric heater after all - absolutely no modification required!!!
What a relief!
Here's the final product: