What would be a modern equivalent to an STM32F103, preferably in a 48 pin package, and which wouldn't require too many alterations to existing code and PCB?

Due to the Great Chip Shortage, any suggested alternative would be hard to find... but perhaps try to start with the 'F30x (see AN4228).

"Equivalency" depends on the particular feature mix you've used. One thing is surely different, the 'F1 has a different GPIO structure than all other STM32.

JW

CubeMX is supposed to have a means of indicating equivalent parts.

I'm not sure the 64-pins devices have been consistent, the 144-pins ones have been more so.

Not sure availability on the F3's is significantly better, the F1's are still made and consumed in large volumes, it's just that all available production is spoken for.

You might do better looking for parts you can source, and then pivot into those.

The STM32G491CCU6 at Mouser looks hopeful.

Moving to another STM32 series is not throwing away years of work. With the knowledge and experience you already gained, moving should take a maximum of few months and you will gain additional features and experience.

Thank you for your answers. So my problem isn't due to the F1 being discontinued, but 'just' the global chip shortage. (Although, if that's the case, why can't I order F1's directly from ST?)

As far as I can tell the F3 isn't any more available than the F1.

The STM32G491CCU6 looks really nice - the RNG would be particularly useful for my product, however it only seems to offer pins for the first 4 ADC1 inputs, but my project uses all 8 ADC1 input pins available on the F103. Is there any way around that limitation?

I'm also imagining a scenario where I spend 2 months converting my project to the G4 only to find the stock of those has fallen to zero - it could presumably happpen tomorrow!

Incidentally, I'm assuming the F103's I had fitted by JLCPCB were fakes, but how can I tell for sure?