The FS450 is off road use only, so if you want a bike for around town, that's not the way to go.
It sounds like you want something track capable, but lighter than the 701, so a conversion might be a decent way to go. It's worth researching your desired power-to-weight ratio, and deciding if saving a few pounds is worth spending thousands to get it right.
Keep in mind, a proper 350 EXC-F conversion will run you in the ballpark of ~$4k, depending how wild you get, how much labor you do yourself, and which, if any, craigslist deals you can find.
A proper conversion will account for suspension valving changes, a new brake setup (4 piston caliper with matching master and >=300mm, >=5mm thick rotor, if you want to keep it on-par with your 701), 17" wheels and tires, and supermoto-offset triple clamps.
Stock enduro brakes will fade fast and enduro suspension is either downright dangerous or mind-numbingly slow at track pace. Sumo offset clamps are optional, especially if you're new to riding supermoto on track, but a sumo on track with enduro clamps will feel sluggish on turn-in compared to sumo clamps.