The 168F is one of the clones of the Honda GX160. Honda claims 5.5 hp (gross) for the original, and the 168F's manufacturer describes it as 5.5 hp but puts a 6.5 hp sticker on it. Remarkable marketing practice. You can get some details of it here:
http://www.alibaba.com/product-gs/999192129/168f_1_6_5hp_honda_type.html

You may be able to get a magnet from one of the importers of the engine. Otherwise a Honda GX160 magnet will probably fit.