What to replace an oil heater from 1989 with in an old witch's house?

Hmm as long as we have the old oil heating system, my understanding is that no additional wood stove may be connected to the chimney.

The question is whether it is single- or double-flued, and then there are differences between chimney sweeps. That was the case here with us... you have to ask them, nothing works without them.

As long as the heating is running and allowed to run and you find the consumption OK, I wouldn't throw away a working heating system for now. Unless you have personal ideological reasons. Of course, it doesn't hurt to come up with a plan for the worst-case scenario and calculate in the meantime what makes sense, which may require longer preparation work (laying a gas connection!). And regardless of that, you can also optimize your personal well-being. For example, if a wood stove is desired, something like that can also be done well with an external chimney. My parents have always heated a lot with wood and that was not a problem as long as my mother was just a housewife. Since the children left the house, often no one was there during the day and the house cools down. The solution for that was a second stove in the living room that quickly warms the room you also use in the evening instead of indirectly from the kitchen. This was solved with an external chimney and works great.

it was an energy consultant based in southern Germany

Great.
That sounds very good.
We live in southern Germany.
Maybe I could get his address?

Then I might open a question when the time comes. But first, I would actually contact a specialist advisor, otherwise I get confused.

Maybe could I get his address?

gladly via PM or you post your email address for two minutes.

There are also automatic pellet stoves into which you can pour a supply. I believe those are also water-bearing. The right choice of heating always involves incredibly many factors. What do I want and am I able to do daily, what possibilities exist for connections, is someone afraid of gas, etc. In the case of the TE I would leave the old heating system. Because switching to a condensing boiler is not financially worthwhile at first. Then improve small things like the cellar ceiling, roller shutter box, etc. Sometimes alternatives may not be real alternatives.

So I would also initially keep the oil heating system, because all the alternatives are not very appealing.

But you could find out how much the gas line would cost without the neighbor's contribution. Either it's exorbitantly expensive (€10,000), or gas is a strong contender to replace oil.

Depending on the federal state, you can easily buy a new oil condensing boiler until the end of 2025. And even after that, you can continue to install a new oil heating system, provided you couple it with renewable energies. Then you would have to see the political situation regarding oil, but I would consider switching to gas sensible, especially considering the costs you save (keyword CO2 tax).

I think a heat pump for an old building without insulation and underfloor heating is not sensible at first glance, and they are also very expensive compared to oil and gas. I also personally have the prejudice that heat pumps are prone to faults, require more maintenance, and are difficult to adjust. Please correct me if I'm wrong ;)