My JMC Vigus has low power and says 'SVS' on the dash

The SVS warning literally means Service Vehicle Soon. It suggests that the on-board computer has detected something wrong with the vehicle, and is warning you that it needs attention pronto.

The fact that it’s losing power is probably evidence of the computer placing it in limp home mode to prevent it doing any further damage to whatever mechanical system is causing the problem in the first place. Take it straight to a workshop and have the vehicle scanned to see what the fault might be.