IIRC the website you need to be checking out is
www.minitwins.co.uk
As for what you can and cant do....basically tuning wise your limited to a full race system, air filter and ECU remap or powercommander depending on your opinions on either method.
Suspension can be change to a new shock of your choice and fork mods providing the externals are the same as originally fitted.
And bodywork you like can be fitted mounted on lightweight subframes should you wish.
wheels have to stay the same along although discs can be changed.
And thats pretty much it bar the usual ACU regs like lockwiring and catch bottles etc etc.
Minitwins really is a cheap for mof racing although bear in mind when i say cheap you are probably looking at in the region of 5k for your first year not including the cost of building the bike.
Any other info you need should be on the website if not feel free to ask here as I know of at least 3 people on this forum who definately know whats what plus me(ish!!)