In a stroke of pure irony I've recently seen a study showing that really old central heating (water based) works quite well with heatpumps.
Here everyone still says that you need floor heating or modern radiators or it won't work. (Here air2air pumps won't get grants because it is easy to use them for cooling...) But it looks like badly insulated pipes in the walls and huge ass cast iron radiators work great.

I have a heat pump and it looses efficiency around 45*, and once u get below zero it just cant keep up. I use my heat pump on the shoulder seasons, but have to rely on my oil furnace for the majority of the winter. If I had the loot I'd get a cold weather heat pump setup as they work much better, but I just spent a boat load of money getting my chimney redone and having a wood stove installed in the hope u can stave off my oil bill with wood ... (Chimney NEEDED done anyways, wasn't a big deal to add wood stove at the same time)

Where did you get that last bit? Mini splits are so easy to put in on old homes. They run the lines externally and make a hole into the wall. I have it in my 1920s home.