In my opinion the OS70 would be a nice choice for that plane. I've owned the OS70(was in another plane), and know they are quite a little power plant. Plenty of punch...and the power band(lots of torque) just might do that Decathlon good. Just watch for ground loops on take off, and keep some speed to it, and land on the mains with some speed. Yes, it's a bit different technique, one that you learn on the fly(pun intended), but know those things going in, and anyone would do fine. Just act on them accordingly .
I also used the Aerotrend(is that right?) foil-shaped tubing for my struts..and still to this day.. ...after several years have never flown it with them..

Matter of fact....here's a quick pic of mine....