How do I fix an old Myson Unit 3 water pump?

The Myson Unit 3 water pump on my system only works on the slow speed now. This doesn't seem man enough for the cold weather we are experiencing at the moment. The pump has two speeds marked I and II on the electrical unit on the top and on the back of the pump it seems to have a lever with speed markings on it as well. Is there any way or servicing this pump to fix the high speed problem? If not, what would be a good replacement pump?