Adam, a few years back I needed an accurate source of drawings for a Sentinel 100HP 4wVBT.
We (the G3 Society) wanted to build an affordable 'entry level' scale model of it. Eventually, we contacted the Sentinel Drivers Club and they kindly provided (photos of) the original GAs, sufficient that I was able to produce good CAD files from them.
I'd suggest that you contact the Club and ask if they can help you too?
Sentinel Drivers Club
Regards,
IanT