Hi,
In the UK the standard set-up for just about everyone flying DF for years was OS91/Ramtec. I think that just says it all...
Mikuk is absolutely correct on the Dynamax thing.
Yes, dump the Rossi - only good for door stops! we tried Rossi engines for a little while, they were absolutely useless! We all switched to OS91's and hey, no problems. We always flew off grass with no drama. As in any other aircraft, keep it light!
Anyway don't stay with DF too long - they are fantastic and so much fun - save up and buy a turbine, you'll never look back!