If you fit a second-hand overhead guard to a forklift truck, would this need to be tested?

Provided that the guard is undamaged, unmodified (including holes drilled for fixtures added post manufacture) and is an original OHG, fitted to the original mounts, with genuine fixtures and is from a machine of the same brand, model and capacity, it should not be necessary to subject it to testing. It will be up to you, or your engineer, as a competent person, to ultimately determine the integrity of the repaired machine.