Are there any simple mods that I can do to my Northstar to make it play better?
Clean the bearing. Run it bone dry or super lightly lubed or Terrapin-X treat it.
Improve your throw.
The Northstar is a really great yoyo and in my opinion the only modification I’d like to see isn’t something I’d like to see in a modification but rather as a production model change: texture the rims like a PGM and make them grindable. Same with the Protostar.
But what are you looking for? Faster? You want a Protostar. More stable? It’s pretty stable. Less vibe? Mine is very vibe free as is my protostar. Stacking would require major surgery. Changing the weight would most likely also require major surgery as well in order to remove the weight ring. Dying? I supposes so, but I’m not sure what material it’s made from but I assume it’s possible. Painting? I suppose so.
Change the bearing? Cleaning the bearing? Well, this requires removing the spacers, which can be tough, but can be done. Removing the spacers can be a bit of a hassle, but it’s not impossible.
Why do you want to mod it? The only mods I can think of for the Northstar are SPR’ing in and beadblasting it. The first requires exceptional skills and the second expensive equipment.
Well mine is just really really vibey
Are your arrows lined up? They tune these at the factory and then MARK them when they find the “sweet spot”. Start there, then rotate them one way or another until you get less vibe.
I’m starting to feel super lucky as my Protostar AND Northstar are both very, very smooth with very little vibe.
Ok thanks I’ll try that and see what happens 
wish me luck!