Phillip the only answer to your question is a massive no the machine would never pass a loler inspection in the uk without the correct specification/weight battery in it and like it or not the truck legally requires a 12 monthly loler carried out by a competent person. The battery in an electric fork lift truck is part of the ballast needed to safely lift the rated load. Fit a lower different capacity battery in the truck and have an accident and your the one in court on a manslaughter charge mate