Hi Steve/The plastic prop might be adequate but usually a brass one looks better if for no other reason! As Jake says you need to know what the shaft thread is – commonly 4mm, but it could be different. The easy way to find out is to try various nuts of a known thread until you find one that goes on easily and that will be your thread. The plastic propeller might have the diameter and even the thread stamped on one of the blades. As Jake said, Westbourne Models is a good place to try, or the Dockyard Model Shop at Chatham, or Scoonie Hobbies. All of these sell a range of kits and have probably had the same question asked before. Membership of a club (if you are not already a member),will be of help to help you. Model Boats magazine has adverts from a number of suppliers, one of whom might be close to you.

try the prop shop, on my Cux87 I use a 3 blade 23mm which seem to run iton a 6v battery pack.

http://www.swangroup.co.uk/propshop.php

Thier product web site link seems to be broken but you could allways give them a call I ahve found them very helpful.