My son and i put a Homelite 30cc on his DVII that we built.All WW1 airplanes,the prop was real close to the cowl.With the 30cc homy in it it still stuck out a little to far.
I think about 1/2 an inch past the cowl.We had no problem with it going in the cowl because its so big and round.The ryobi would be too long unless you modified the firewall.Wich will be bad because thats where the front landing gear cross leg almost ends up.

Check the plans.Even the short shaft would be too long i think as i have a long shaft for sale in the classifieds here on rcu,and i have a short shaft on my shelf yet.
It had stock intake manifold,pitts side dump muffler,stock carb,mag ignition.I could hand start it on the 3rd flip every time.
No mod to firewall,cowl or anything to fit.

Running a zinger 18x10,7200 rpms about.Took off fine and flew most of the time around half throttle.I think my sons came out a little heavy.I asked Balsa USA if they would weigh the box and send me the lightes one because i was doing my own engine.They said they all weigh within a few ounces,because of there choice of woods.I doubt it.
Anyway,very nice plane and combo.What do you mean by Zenoah 41?Maybe a US engines 41,quadra 42.

A Quadra 35 up to a 41 would be killer.Im putting a quadra 35 in my 80 inch Jerry Behrens scratch built Sopwith Tripe.