The 525 motor requires modification to the lower frame mount if putting it in a SX frame. Also make sure it's from the straight axle machines, not the IRS model. Can get a lot of power from the RFS (450/525XC) engine platform, but it's just different. They'll never have the top end overrev that the SX engines have.What about a Polaris 525 motor from the later models? not as good as a 505 but you’d get reverse and you can build them up pretty easy to make a lot of power.
See them going cheap quite often(compared to the KTMs anyway).
And wouldn’t require fabrication of mounts and who knows what else, unless your into that...if so than o vote 300 2 stroke! Lol