The reason your friend says it's a 175 is the physical size of the cylinder & head, I haven't actually seen a CZ 250 in the flesh but the Jawa motor is quite a bit larger in the cases & topend than the 175s
The 125 & 175 CZs & Jawas were the same motor with the outer covers having the different symbols on only, can't quite recall if the model numbers were the same or not, seem to recall Jawa used 3xx while CZ was 450,453 for the roadbikes & 470,473 for the trail/farm bikes
He is correct on the forks being from a square case & cylinder bike, would need to be changed for pre75