Howdy. I guess the closest match in the UK & most readily available there would be an engine from a Kawasaki GT750 (KZ750P).
Cmsnl.com has the diagrams for some of those models. Look in the Kawasaki motorcycle index >> Z750 models.
You can compare the parts to the 750 Spectre models which are listed under KZ750. The Spectre was sold in the USA, so most parts diagrams will have the model. Cmsnl is the only place I know of that has the GT750 diagrams. unfortunately, we didn't get that model over here.
ps: you might want to dump the Spectre & get a GT. It's a neat looking model IMHO.