The replacement parts is a tricky assessment. For instance, if we are talking brakes then one would say ...

Brompton: from a Brompton online/local dealer
tikit: from any online/local shop that offers a v-brake

If you are talking about something specific about the folding mechanism then the tikit would be more isolated.

Although I should add that Brompton parts/dealers are not so widespread here in the US. Brompton part availability can certainly be different elsewhere in the world.