I can't offer advice on how hard it would be to change that particular motor, as I've not done it. Similar changes have not been overly difficult however.
First suggestion would be to contact NMA they ought to have the answer to the spares situation as the Kity importers. Iinstinct tells me this route is going to be expensive. My first look would be to see if the mounting is fairly standard, and if there is a general after market motor available with the correct speed and shaft size. Machine mart or axminster do a range for instance. I am sure there are other options that members with more experience will offer soon.