You could buy a RL cart from Mojo and get a competent shop that they trust to fit it.
Or send it to Mojo for a service and get them to fit the RL cart.
If the fork is only about a year old, a Mojo service (annual service) would mean you get a second year of warranty on the fork. AS long as the maintenance schedule has been followed and the stanchions aren't scored.
The you get a 140mm fork and 12 months warranty for £99 + the cost of the RL cart.
Or get ripped off by someone trying to swap an inferior 140mm used fork ad lose money on the deal.