I hadn't really listened to my Monitor Ones in awhile. I had them loaned to a buddy of mine with my modded RA-100 amp and was over at his place checking out some mixes he had been working on. I was struck by two things...
One...The bottom end was surprisingly good. Better than I remembered.
Two...The top end was ragged and edgey. Not smooth at all.
What do I remember about my mixes on those things? Boomy bottom and subdued top. Why? Probably because all that spit and distortion from the high-end of the monitors was making me turn it down.
That said...I don't think the crossover mod alone will get you to where you need to be. I've heard a similar Vifa tweeter to the one I linked and thought it sounded really good. Both Blue Sky and Lipinski have used a version of it in their monitors so it's in good company.
Look here and see if there's anything in here that makes sense: Zaph|Audio
One thing to keep in mind...The tweeter, woofer and crossover work together as a system...Change out one component for another and the whole system behaves differently.
I have no doubt that replacing the electrolytics in the crossover will make things sound better. Maybe that will be enough for you. But to go further down the rabbit hole with a new tweeter will require some math.
There's quite a bit of learning to be had from that Zaph site. You'll pick up alot by just thumbing through it.
Keep us posted on what you do and your impressions. I might just join you on the journey.