If its a 'drop-in' it should be a kit that includes a custom muffler (or custom exhaust pipes), longer drive shaft and custom motor mounts, and all the electrical harness that it takes to convert it over. (Onan didn't have a fuel solenoid. Briggs does. Onan had battery ignition, Briggs is magneto ignition.)
The Onan is larger than most modern engines of similar HP. The other guy is right, the Onan had lots more torque at 18 hp. than some of these EPA engines do.
If you are getting an engine in a OEM crate and that's it, its not a 'drop-in'. Regardless of whether its a Briggs or a Honda. (If you get a Briggs, it better be a Vanguard.)