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Well, all you have to do is get some trailwing tires or similar. But full height dirtbikes are not easy to flattrack. It can be done, but it's just not possible to control the big leaned over slides with such a high COG. Also, it's not confidence inspiring and will slow the learning process.
CRF 150s are perfect to learn on. I would keep the 350 as is, and get ice tires for it (ice is more forgiving), but get a little bike for flattrack.