Th e "hard gunky stuff" maybe Loctite if the thread was stripped or damaged. The hard locking type can usually be loosened with heat. A small butane torch will do the job if it is that. As said drilling out shouldn't be a problem but you need a GOOD drill bit. Ones from cheap sets don't usually have what it takes. Not easy to get a good hold of the drill and push down straight either. Filled flat. a good centre punch dead centre should enable you to use 2 sizes of drill to get you right out to the thread. A needle file will make a cut to let you pry out the thread that remains. If you find that the thread in the head is damaged and unusable a helicoil is best if you can borrow the tool.
Get new bolts for your pump. Take one of the old ones and hack saw it lengthwise down through the thread to the smooth part. Run this in and out of the holes to clean out the threads. Use some of the penetrating oil on this.
Obviously if you have a suitable tap, use that.
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