Considering its a Nova Based motor. You probably have the M4-R? is that correct. Your best bet is just going with Nova parts. Many of the good engine builders here would be able to help. Possibly contact Novarossi Direct here in the USA.
I am not sure but it may be related to the Novarossi 21-4 motor. You have to look in your spec sheet to see if this may be the case.