Just been doing some quick reading up on this, and it seems to be a very muddy area...
Basically you need 8 points from the table below (and the chassis has to be 5 of those).
If you can't add up 8 points for your car it's classed as radically altered and you need an IVA and a new registration.
What isn't entirely clear is what level of chassis modification is allowed before it counts as no longer being original.
If you're just adding a couple of extra brackets to the chassis you're probably ok.
If you start cutting bits out of the chassis you're probably looking down the barrel of an IVA.
On a kit car however I suspect it's going to be very hard for anyone to prove or disprove which bits are original and which aren't, and I doubt anyone would even try.