For 24 bucks I would just get the whole thing. It comes with 3bb blade grips, spindle, stepped washers, regular washers and 4 of the head dampening shims.. The upgrade is super easy.. you unsnap the paddle control frames, remove the blades, take one of the screws out of the spindle and slide the whole thing out.

Then just reverse the steps and put the hiller head on. If you have ever replaced a blade grip this is almost an identical procedure, very easy.. just pay attention to what order the parts come off in, and put them back in the same order. I would reccomend putting all of the dampening shims in, since serious cyclic is the point of this upgrade, you might as well stiffen the entire head up while you are at it.

If you did want to upgrade without buying the 25 dollar head you would need the grips and the mixer arms, and those would go on your existing spindle shaft.