A lot of good advice has been given in this thread. Attempting to drill a stainless bolt for a left hand threaded remover will not be fun. If I have part of a bolt sticking up like that, I usually go for the vice grips first. Use a small cutoff wheel to make the slot for the screwdriver if the the vicegrips don't work.

A thread chaser is what you really want, not a thread tap. A tap cuts new threads, and chaser restores damaged threads. I have the set below and use it all the time. I often run a chaser through questionable aluminum or rusty steel threads before reinstalling. Saves many headaches.

If the threads are too far gone, then, yeah, helicoil.

Craftsman 48 pc. SAE & Metric Thread Restorer Kit