Little bit confused by Your post ?
Are You having difficulty with access to repair the missing fin , technique,or contact details for a repair specialist ?
Best method on the aluminium head is T.I.G in My experience , M.M.A for cast iron , however "Brian" guided Me recently in repairing fins using M.I.G with good results .
Pre-heat & post-heat may be required .
To reach into the root of a missing fin part way down the head for example usually requires careful removal of a fin above for access , working upwards or downwards as the case may be replacing removed fins subsequently .
I fabricate missing cast iron fins in mild steel leaving an extended tab for coping , this can be clamped or wedged to retain the fin in correct plane .. ground away after the repair is affected .
Hope the above may give You the "bare bones" of a method of progress ?
Regards,
Steve..