as I have no knowledge of what is available other than Hornby chassis.
Within the many thousands of contributions in this forume almost every available drive or chassis has been used and tested.
So it is very easy, to gain the nessessary knowledge. The search function of this forum might be helpful.
If somenone is asking for a suitable drive for a model, the least I´d except, to get informations about some of the basic dimensions.
I can not really believe, that a 16mm scale Simplex still fits in the category of Gn15. Better in "Seize does not matter"
Be careful when asking for cheap
solutions. Have a look for the cheap solutions proposed in here in the past. Most of them do not run very well,
tend to stall or race along the track. In many cases, you never here of the cheap stuff any more.
Look for quality. In the long run it might cost you a few Pounds, Euro, Dollars more, but you´ll have a chance, to enjoy them for a very long time.