This might be a question better answered by Dan or Lynn, but I will give it a shot…
If you want to replace a missing aircraft ID plate legally on a certificated aircraft you will need to get the FAA involved. This situation is specifically discussed in FAA Order 8130.2J, section 23-5. Here is the language:
23-5. Replacement ID Plates.
a. When a replacement ID plate is required, the owner or the owner’s authorized representative will contact their local FAA office. The FAA determines whether the request is valid and provides a letter to the applicant with the FAA’s finding. If the FAA finds that the request is valid, the applicant includes the FAA letter with their request for the replacement ID plate from the appropriate manufacturer.
b. Upon notification by the applicant, which must include the FAA’s letter, the product manufacturer may then issue the replacement ID plate.
c. The old ID plate, when available, must be voluntarily surrendered by the owner with a written statement to the FAA office that authorized the replacement. The FAA office must make a copy of the plate and then either physically destroy it or, if requested by the manufacturer, return it to the manufacturer via certified mail. The FAA office must then send a letter to AFS-750 stating that the surrendered plate has been destroyed. AFS-750 will include the letter in the permanent aircraft records file.
I doubt very much that Alexandria Aircraft will want to get involved with this since they do not own the type certificate for the Cruisair. Therefore as part of your petition to the FAA you might receive their permission to have a new plate fabricated on your own. This would be easy as you could buy a blank plate from Aircraft Spruce ( https://www.aircraftspruce.com/catalog/cspages/acid.php ) and simply take it to a trophy shop for engraving.