Cast iron toilet stack

The good news is you don't need a special flexi that fits 160mm — you just need the right adapter. Here's exactly what to do:
What is happening
The 160mm you see is the outside of the cast iron; the actual internal opening is almost certainly 110mm — standard size. Old cast iron often looks huge from the outside but the hole inside is the same as modern plastic pipe.
Solution 1 — Check the internal size first
Clean away the debris and push a tape measure inside the pipe opening
If it measures roughly 100–110mm internally, a standard 110mm flexible pan connector will push straight inside and seal perfectly — use the finned rubber outlet type (McAlpine or Fluidmaster) — these are designed specifically to seal inside cast iron
Solution 2 — If the internal opening really is 120mm
Use a flexible rubber boot connector (also called a Fernco coupler or flexible donut adapter) — one end fits over the 110mm pan connector, the other end stretches to fit the larger cast iron. These are available for 110mm to 160mm transitions
Alternatively, fit a purpose-made cast iron to plastic adapter — these clamp onto the cast iron and give you a standard 110mm spigot to connect your pan connector to
Solution 3 — Reduce the opening
If the flared part is too big, you can pack the gap with a sleeve of 110mm pipe pushed a short way inside, then seal around it with a flexible rubber banded coupling
Important
Do not try to fill a large gap with sealant only — it will leak
Make sure whatever you use pushes fully home and maintains a clear 110mm bore right through — any restriction will cause blockages
Summary
Measure the internal diameter — it is almost certainly 110mm
If yes ? standard 110mm flexible pan connector with the finned rubber outlet pushes straight in
If genuinely larger ? use a rubber reducer adapter (110mm to 160mm) widely available from plumbers' merchants

This is a very common issue when dealing with older cast iron soil stacks.
Flexible pan connectors are generally not designed to seal directly into cast iron, especially where the internal diameter is irregular or flared, as in your case. This is why you are struggling to find a suitable flexi connector to fit properly.
The correct solution is to use a cast iron to plastic adaptor, often called a CI to PVC soil adaptor or a rubber compression adaptor. This type of fitting is designed to clamp securely onto the cast iron pipe and provide a standard 110mm plastic outlet that a modern pan connector can then connect to.
In some cases, the cast iron opening may need to be cleaned back thoroughly and measured accurately, as old stacks can vary in size. A rubber adaptor with a wide tolerance range is usually required.
If the cast iron is badly corroded, cracked, or out of shape, the best long-term solution may be to cut back the cast iron stack and convert to plastic at a suitable point using a proper conversion coupling.
Avoid using sealant alone or improvised fixes, as these will almost certainly leak over time. Using the correct adaptor will give you a secure, serviceable, and regulation-compliant connection.

In this instance I personally would source one 160mm coupling reducer from 160mm to 120mm and fix this in place to the old cast iron stack and then cut a piece of osma drainage pipe to insert and then connect to to a flexipipe designed to attach to a toilet and also the osma pipe

I would suggest cutting the cast iron stack from the outside of the building and replacing the removed stack in plastic. Future proofs any further adjustments you may make to the toilet.

You may require a rubber reducer like the one below, or similar.

https://amzn.eu/d/iTE3t3v