Hi Nick,
everyone is no doubt on the panic to get ready for Mallory at the moment, it may be easier to meet everyone in the paddock and discuss this with them, as machines are rarely offered in the forum, and good examples, ready to go, are difficult to find.
I can put you in contact with a New Zealand ex-bantammite who has a good 125 engine that would get you started.
Email me via the forum admin.
All Best,
Alan