Damn, that is a major bitch!
What brand/quality level tap are we talking about?
Quality taps are harder then the hinges of hell, so you aren,t going to be able to drill it, not even a little
... don't even try, it'll only make things worse ...
Cheap taps can be fractured in place, so knowing what you're working with will help folks give you the best advice.
At any rate, the best thing you could do is take a step back and look at the big picture.
Were you able to re-time the t-belt?
Might be a good idea to see if anything in the valvetrain got hurt before procceeding.
... it would suck to get the crank bolt sorted out only to find there's sh*t bent up top.
You don;t have to have the bolt in the crank to do this task.
When an engine has lost time, I like to get the crank out of the way before spinning the cams.
Set the crank up so it's at 90* to tdc
Spin the cams, one at a time to the dowels up position,
Now, sneak the crank up to tdc, and set up the timing.
You can't spin the engine on the starter, (belt might walk off), and with no crank bolt it will be hard to move things
... but you can turn the engine over by installling the alternator belt snugly and then use a socket on the hex bolt of the alternator pulley to spin the whole shitaree over.
Once you get that task handled, you can do a leak down test on each cylinder in turn (just rotate everything using the alty nut)
If the motor survived with no bent valves, then you can worry about yoking the broken tap out with the crank in situ.
They make a special tool that goes down between the flutes and it will be your best bet.
I repeat; DO NOT TRY AND DRILL THE TAP OUT
With a diamond burr and a steady hand, there is another way, but it's a hell of a lot of work
Knowing if it's hurt up top will help you decide how to approach this.
... if she's gotta come apart, you can have the broken tapp e.d.m 'd out "easy peasy" at Colorado crank or one of the other 'grinders down in town.