[woodcote]

I would try the Scalextric Hot Laps app (Android & iOS). You'll need to build a gantry so it fits over the track (Lego is ideal, if you have some handy). There's some decent gameplay in there too.

Another iOS option is LapTracker - AutoTimer, which is a generic timing app for various hobbies, athletics etc. An Android equivalent is Laptrax.

With all these, you need to be careful with lighting and shadows which can trigger a false lap. I think Hot Laps is the easiest to use, although I have used LapTracker for years for practice sessions at home - and timing proxy races.

Otherwise, it's a bit of a jump in complexity and price.