The engine in this first 1906 model of Ellehammer was just 9 (nine !) hp. His first flight with this machine was made on
September 12, 1906, where he flew for 42 meter.
As you asked for a suitable engine equivalent to the contemporary 9 hp engine (probably a modified motorcycle engine, but I have not checked that). Being not an expert on todays (or yesterdays) engines, I was surprised that even ultralight engines already did 35 till 50 hp
A fine one - but probably excessively expensive - and surely too heavy is a late age rotary engine, which can be found
here.
Perhaps you can use a modified lawnmower engine, which also develop almost 10 hp.
Sure there are more engine knowledgeable people on the Forum.
Have a good time
Kees