There is a spring, hope it has been retained in the drive casing rather than lost. Remove HT cap from spark plug for safety. Tip machine on its side - left will flood fuel system, right allows oil to go where it's not helpful. Remove blade - 17mm. Remove cover/guard - 8mm, three bolts. This accesses drive belt/gearbox. Tension spring is about 35mm long x 8mm dia. with two hooked ends. One hook fits in gearbox casing. Other hook goes into hole in mower's sheet metal casing. Probably easier to install spring before trying to attach drivebelt. Refit all bits removed for access, tip mower upright. Let stand for ten minutes if carb/filter has flooded. Refit HT. If spring is lost you may be better off removing drivebelt and using as an un-powered mower. Also, if the drive belt cover is smashed there's little hope of trying to continue using drive mechanism without cover as mechanism just gets gunged up with grass and belt continually comes off.
Goz
August 2008