Custom built stroker. You could have a stock crank stroked, but you’d still have to use the Hot Rods rod to keep things fairly simple. Otherwise you can do whatever you want, but you just have to find pistons with the proper deck height to accommodate the stroke you want to do. Then you’ll have to have the rod clearanced and clearance the cases so it fits. it’s quite tricky and you’ll have over a grand into a crank real quick.
There are “drop in” 4.2 cranks available that use an OEM crank and then are stroked using a custom rod to minimize case clearancing, but they are still pretty custom and you’ll have to use a different piston like a CRF450R with a 19mm small end and a shorter Compression height. And you’ll need a custom cylinder since there are no 94mm CRF pistons. And you’ll likely need you head decked to get back the compression you lose from having a bigger combustion chamber in the 04-05 head. They are sweet motors, but you’ll have a couple grand into just those parts.
And this is all assuming you have an 04-05 machine which I have no idea since you don’t mention it or have your signature filled out. If you have an 06+ the only option is the HR cranks, unless you wanna do some crazy stuff.
I sold a hundreds of +3 crankshafts from Hotrods over the years to recreational riders with only a handful of issues. If you're chasing the podium, I suggest sending the crank and piston to crank works for balancing to improve durability.