Basil:

Read the instructions, the hardwood blocks are to mount the cowl. You need to pin the firewall for strength for your engine, as per the instructions. If you do that the DA50 will be fine for that plane. I use a BME50 on mine, basically the same engine and no extra 'beefing up' was needed.

Just follow the instructions and use what Great Planes gave you and the plane will be fine. My only recommendation on changing anything is the tail wheel. Although the one GP gives you is fine, it tends to fall apart after awhile. It is (in my opinion) way too small.