I've not seen it modeled before so looks like its a case of getting some good square on photos, picking the size of flywheel you can manage and them proportioning everything else from that. It does not look too had a one to fabricate, you could always use something like the Vulcan beam as a basis to get the main working sizes and then detail it to represent this engine, the compound cylinders would make it an interesting subject.
Preston's also have a Woolf compound beam engine so you may be able to get some details from that too, they steam it on their open days, I took these at the last one.
I can't see any more photos of the Woolf compound on Preston's website. However there's an 1891 Easton & Anderson A-frame Woolf compound at the Forncett Steam Museum in south Norfolk. If you are anywhere near this part of the world, it's regularly in steam on the first Sunday of the month through till October. There are some still photos of it on Youtube at https://www.youtube.com/watch?v=ZYvbor87Ijo – from 2mins 12 secs.
Looking ahead, I'd love to see a build log when you make the model.
I have an idea that The Museum of Power at Langford have some connection with Beeleigh as they are only a few hundred yards appart. I am sure that if this is the case you may be able to gain access to the engine for photographs and dimensions. A phone call could prove usefull http://www.museumofpower.org.uk
Thanks for all the comments and advice, i shall be writing to the restoration people in due course. Some of this engine seems to be underground so that will be a challenge !!