Be sure to check the pads and rotor for contamination especially spilled fork oil. Even if the rotor appears clean or has been cleaned there could be oil soaked into the pads.It's been my experience that there is nothing more effective then the OEM front brakes. If you're having problems you need to look at the master cylinder, the pads, the fluid and the calipers. The original brakes should be able to stop the bike on a dime.
Yeah, HD lies on their 41mm forks... they’re actually a true 1.625” or 41.275mm. You could potentially hone the factory Honda trees out to that diameter and figure out Harley lowers, a front wheel and brake/tree widths but that seems like a hassle. Harley parts are cheap though!