An easier modification would possibly be to change the rocker arm ratios. By enlarging the pivot holes in the rocker arms and putting bushings in them you could relocate the pivot hole slightly more towards the pushrod. This would give you more valve lift and slightly more duration. Otherwise it wouldn't be too hard to make a new cam if you have the right machinery available. You would need a cam grinder and a gear hobber. Also, you would need to understand heat treating so as to surface harden the lobes before they are finish ground. There are lots of machine shops that are capable of doing this kind of work but the problem is finding someone that will do it for cheap. 4-stroke cams are quite complex and the only way to make them cheap is to be able to mass produce them and sell them by the thousands, like Saito, Y.S., Thundertiger and O.S. do.