P0137 code and recommended mappers around wiltshire area?

Hi all

I have recently bought a forester xt in black which has been made into an sti replica (sort of) and has had the sti suspension and engine and brakes from a hawkeye impreza sti, which I believe is the ej257 engine?

I'm not sure whether the ecu it's running is the original or from the Impreza and has had a decat pipe fitted so kicks up the engine management light and gives code p0137 as the lambda is not connected.

I tried the resistor mod with no luck, so am looking for advise whether there's anyone around the wiltshire area that would be able to remap to remove the code ever appearing and also so that it will happily pass emissions for mot and any performance gains if there is any to be had

Thanks Marcus