Search using google and see if a model of the engines can even be found. If not chances are you will be the first and have to draw the engine and design to what scale you want to model in. Most of what you see on this site is a result of someone being the first and doing just that. After that a small percentage will become available as dimensioned drawings as even that is a huge amount of work. I hope you have good luck on your search. I would like to see one or both of the two built. I like both engines.
An air-cooled 4 cylinder in-line engine is a rare item amongst hobbyists. There is one that I have seen plans for in about 35 years in the hobby. It is a model of a 1930's British aircraft engine called a De Havilland Cirrus MK. 1. A complete plans package can be found in back issues of Strictly IC magazine. Go to http://www.strictlyic.com/ser01.htm Scroll down.
The book Early Motorcycles Construction Operation and Repair by Victor W Pace might help you. There is a drawing of the manifold side of a Henderson 4 cylinder on page 132. On page 133 there are two sectional views of the Henderson. With the sectional views and some imagination, it should be possible to develop scale drawings for the Henderson.