To be legal to run in BRCA affiliated race meetings a full rolling chassis kit has to be available, and that has to be available for sale for no more than £99.95. This means as a conversion that uses a different chassis it would not be legal for these events.
That doesn't mean it wouldn't be legal for other events or in other countries though.
A full copy of the BRCA rules is at
http://www.brca.org/sections/subsect...ologation/1142