Post#2 » Sat Oct 27, 2018 12:51 pm
Steering head specs are remarkably standardized...there's a decent chance that a front end from any given MX bike might just go on - but it would really throw off the geometry of the bike because they'd be so much longer, and you'd certainly have a totally different rake/trail on the Sportster frame than what was intended on the MX frame.
Realistically if you want to be "serious" about hillclimbing your Sportster, the path of least resistance is to get a MX bike of some sort, and do a motor transplant...then mod it's suspension and swingarm length to taste.
If you don't want to do something like that, probably your best bet is to get your existing suspension to a proper suspension shop and have them do what they can with them. Assuming you extend the swingarm, the rear shock will need a new spring and valving...and granted that you're going to be getting more air than the typical Sportster is intended for, the same thing will apply to your forks so they can respond to jumps on the hills.