Hello Aiden
This is the bigger Villiers Mk25C 2-stroke.
Bits are still available.

An engine swap would be the last thing to do - for the simple
reason that the engine mounts are unique to this engine.
You would need to modify the base mounts to accommodate a modern 4-stroke.

A 4-stroke will not bolt on; neither will any other 2-stroke in any simple way.
It would be a shame to lose that connection - between mower and original engine.
That is a personal view.

If you know engine theory, and the engine has compression and spark,
then the fuel mix must be fresh. It is possible that the engine is miss-timed.

There are videos on YouTube to time the 25C.

Hope this helps.
-----------------------------
Jack


Last edited by CyberJack; 07/01/18 08:45 PM. Reason: Added thoughts.