Replace oil heating with heat pump or pellet heating? Alternatives?

Hello, in our house built in 1978, the oil heating system is faulty; it still works but no longer reliably. Therefore, a replacement is necessary. Question: What is a sensible alternative? My initial research has shown that a heat pump is not economical if no underfloor heating is installed. As few modifications as possible should be made. Is a pellet heating system then the only option?

What was renovated? Radiators are not per se an exclusion criterion for a heat pump.

My initial research has shown that a heat pump is not economical,

How much oil have you used on average in recent years for heating and hot water, what flow temperature was necessary for the radiators, please upload a photo of a radiator.
Then it can be assessed a bit more accurately.

This is my parents' house. I still need to inquire about the exact data. Due to the current situation, I am first trying to get an understanding of the topic. I already thought, as described above, that radiators are a disqualifying factor for a heat pump. In general, the question for me is: What alternatives to the oil heating system are being considered if a replacement is needed and no major renovation work is desired. Perhaps one then has to resort to the oil hybrid heating system. As I understand it: Oil heating will still be allowed from 2026 if a renewable component is added, e.g., solar thermal energy. But honestly: With the existing house, how is that supposed to be possible so easily without major interventions? For example, there are not even any pipes or shafts to get from the roof to the heating cellar.

How much oil have you consumed on average over the past years for heating and hot water, what flow temperature was necessary for the radiators, please post a photo of one radiator. Then it can be assessed a little more precisely.

These questions are already answered directly by the year the house was built! (Hardly any modernizations)

In general, the question for me is: What alternatives to the oil heating system are being considered if a replacement is needed and no major renovation work is desired?
Maybe one then has to resort to the oil hybrid heating system.

Unrenovated with radiators: oil, gas, pellets, all are possible as hybrids with heat pumps. However, it will certainly not be cheaper than before.
Quite honestly, I would consider investing money and renovating right away to reduce consumption; then a heat pump can also be suitable.