I just got one of those engines too, lol!
I'm putting that one rack mounted on the bike. It would be great for a trailer.
For a trailer, the 2.5 would be easier than the 6.5HP IMO. The 6.5HP breaks stuff from the torque, and is really heavy, so everything has to be built really well. You can get away with more with the 2.5HP, and it may be easier to control. I got a #41 chain 5/8 shaft-clutch for mine, so it's strong for the engine size.
Hilliard Extreme-Duty Centrifugal Clutch and parts
Have fun with your trailer! Post it here when it's done!!!