Hi Jack
Sorry to hear its still not running right..
When you say rust ...this will just be the fly wheel weight unless its been taken off...
They are all prone to rust up with any hint of water etc etc.
Do NOT use WD40 (Use a contact cleaner) LIKE IN LINKY http://www.putoline.com/en/products/catalogue/segp_kart/pmp/540/karting/maintenance-products/contact-cleaner-spray-ae/
WD40 will only let any rubbish (DUST /DIRT) stick to the electrical components and build up all sorts of crap
Have you checked the bikes kill switch ? (even on a new bike the beta ones are not the best) if its been on the bike for a long time it may be faulty.
Is the earth lead on the bike clean and fixed down properly ?
Check the HT lead at both ends
If all else fails it may be to strator problem... (i would phone lampkins /beta uk importers 01535 655970) and tell them your problem and they will point you in the right direction. (they have a very good after sales reputation)
Cheers