De Stroke kv6 2.5l too 2.0l

Is it possible too de stroke 2.5 kv6 too a 2.0litre as I want too run my ZS180 in a under 2litre class but don’t want too swap too the k4 with too many mods too be done. I know the BTCC cars did it years ago what do you have too change ie stroke bore, pistons etc which would be the easiest option?