Undo bolt securing blade and remove blade then undo the 8 screws showing on the underside of the flymo (may have to remove any caked grass to find them all) then motor and attached drive belt cover can be removed.
Do you know where a cheap replacement motor can be sourced? Cheapest I could find on the internet was about £40 with delivery.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.