I believe the Robin/Subaru engines have "John Deere" quality. I have a 1.6hp 35cc R/S that served well on my first MB. I also have a new 6hp overhead cam Robin engine($105 ebay) being installed on my B.O.B. trailer.
Roadkill, don't use 1.5" roller; it's made for mid-high speed and low/midrange will suffer.
The absolute best pulling/towing solution would be Staton/NuVinci chain drive and eho35 engine. Unsure if you can afford it and handle the installation.
Find an EXTRACYCLE assembly; this is EXACTLY what you NEED, with or without engine.
For your application you need ALL low end power/torque with max speed 15mph. This will allow you to EASILY pull 300 pounds uphill with 7-speed pedal assist.
Get the biggest tires you can afford. If using rear friction drive, use knobby tire in the front, not in the rear.
If NuVinci is not affordable and you already have the bikeparts.com friction drive assembly, maybe I can talk you into converting your friction drive components to chain drive?
Since hauling heavy stuff usually begins or ends up in muddly, grassy or gravelly roads, methinks friction drive would not be a reliable power train.