very strange situation, ive heard of an xr4x4 engine specialised built that was not fit for purpose and seem to remember a few xroc members jumped on the bandwagon with tales of woe.
if i remember correctly you need to write to him giving him the opportunity to rectify any faults at his expense before you go legal. a cleverly worded letter does the trick 9 times out of 10.
if not ask kosman to bum him.