iirc when young he was playing PG.
He still has good court vision and dribble.
Problem is he does not have the fast footwork required for the position, and I can't blame him, he is 7ft tall. Even players with better fundamentals than Kelly (Pau, Dirk) did not play SF. Those who did (Kukoc, Giannis) are at a completely different level when it comes to dribbling, passing, speed, explosiveness etc.
And even if this works offensively, I cannot imagine Kelly defending a SF.
I think playing as stretch four fits him much better.