Yes most of the race bikes had a Spondon frame but I am restoring the Harris framed bike that Ron Haslam rode at the 1991 British GP. They changed to Harris frames when Barry Simmons took over from Brian Crighton.
Do you have any details for Pearsons Tanks ? I am in Australia so any contact details especially email address would be great.
This is the bike I am restoring http://www.jpsnorton.com/photogalleryviewer.asp?image=nrs558003
Thanks
Ashley
IIRC the JPN rotary bikes had a Spondon Frame rather than Harris. They are
based in Derby and normally pretty helpful.
If all else fails you could probably get one made from photos by Pearson
Tanks (based in Dartford, Kent).
Good luck!