Do you mean the Mk111 Rover style heater Kermit? Or a Clubby one?
I reckon the Rover unit will fit on the basis that plenty of UK Mk1's (including my Traveller) came from the factory with a Mk1 Cooper S heater and a magic wand.
The main casing of the Rover heaters is the same size as the Mk 1 Cooper S heaters. It's only the control panel and associated levers that differs significantly.
I fitted a Rover heater to my old van and it bolted straight up to the lower dash rail. All I had to add was a couple of brackets on the firewall at the back of the heater. I wasn't happy with the way the heater controls stuck out from the dash rail, so I fitted a Rover lower crash pad too. Unlike the Mk1 heaters, the Rover heater hoses exit the wrong side of the case. I passed them through holes in the fresh air duct blanking plate. You could just run a loop back over to the redundant LHD cluch and brake holes, as per normal Clubby/Cooper fitment.
You may also need to remove a welded-on duct on the drivers side behind the dash trim so you can fit the demister ducts.
Cheers,