While welding may cure the pinhole, it may not, and there may be more just waiting to corrode thru the thin. So you want a new casting, since this one is obviously faulty, and you'll reuse the valves and springs (new seals) and hardware and rockers/shafts. Get new guides.

Re the cam. Most likely an install mistake. You need to load up the cam lobes and rocker slipper and valve tip and HLA end with cam break in lube, or it will eat up the lobes. Sometimes, if you reuse a cam with a new rocker, or old rocker and new cam, the old part is already pattern worn to match it's old mate, and is quickly wiped out by the new part, once the hardened wear layer is worn thru.

That way, you are out a new cam and rockers and seals on a new casting from them, and it's a fair deal both ways.