...A lot can be achieved with a little single cylinder flathead. I have no Idea what the Jr. Dragsters do...
If for example, you reverse calculate the power based on the 7.9 second eighth mile capability of a 500 lb junior and extrapolate that into a 12.3 second quarter mile time then calculate the power to do that on his $4800 3 3/4" bore single you'll get roughly 50 hp from a 25 ci engine , or TWO HP PER CUBIC INCH.
http://www.stoutracingengines.com/jrdrag.html