If you know the order number for the previously purchased software, then you can visit the link posted below to check it. If it's still valid, various details will be shown instead.
You can then use the order details to contact the developers. They should provide the original serial number to activate it.
Message for the lost registration code: "If you have lost the unlock code due to computer crash or virus, please send your details to firstname.lastname@example.org. Include full name and home address specified in the order form."