Lightened crank, clutch, brake disc, drive shafts, track, everything what rotates with engine. I can feel when rotating mass is against me, but it is only when rpms are very high. I cant feel it an issue, it just calms sled behaviour in high speeds.
First of all, change your front end with at least 3" forward. Then you can control your sled and change directions and have same kind of feeling like rmk/m sleds.
Second, lowering the mass center of your sled, like removing upper radiator and lighter battery, "smaller" fuel tank or something that is not so high than stock.
The front end is really an issue of this sled when boondocking. Turboed sleds have easily 30-50% more track speed when hitting wide open, so it really has more rotating mass in same situation than "normal" sleds and always when track speeds are high, but I can't see that an issue, it is more like positive issue. When climbing with nytro, skis don't usually touch the ground so then there is only rotating masses and you. Adjusting handlebars might help a little, maybe different riser or just shape of handlebars is answer, but it still wont get anywhere from unpredictable front of nytro. I mean, even with turbos, sometimes skis have to come down..
If you still feel that rotating mass is an issue, get/make nypex, then there is two rotating mass against each other and sled will be something else to drive. I would like to test one sometime..