You would have to replace your whole exhaust manifold, Depends on how lucky you are at finding one of those... I would try lots of penetrating liquid and an extractor bit first, with a lot of patience of course.

 

Will these guys work?

https://www.mcmaster.com/93805a355

 

@originalpy you can drill them out and re-tap in your garage or a shop it is actually pretty easy.

You have 2 different studs - 2 can drill through and change to a bolt the other you can not.\

The biggest issue will be getting the manifold off. That takes time and patience even if the bolts break loose easy.

You will need to order manifold bolts and gaskets yes replace the bolts
A 12 point Deep and 12 point Regular 12 mm socket
A 3/8th drive ratchet
A couple different extensions
A tap M8 x 1.25 metric - Bottoming if it is the one the does not go through

Biggest to thing remember you are working with an aluminum block more force is not better take it easy, no big cheaters on the ratchet

Manifold - start with penetrating oil for a couple of days
The same with the Y pipe bolts

With the engine warm to the touch - not hot your hands will be in there try crack each bolt lose - do not go any further until they all get lose
Now go underneath and do the Y pipe bolts - pull those out. If they start turning and get tight - more penetrating oil then tighten and loosen until they come off

Now remove the Manifold bolts, bottom 1st then the outside top and work to the inside top, the same rule as the Y Pipe, if they start turning and get tight - more penetrating oil then tighten and loosen until they come off.

Each manifold bolt has a spacer, put them somewhere safe

It takes some maneuvering to the the manifold out, but it will come out.

To drill the stud is the hard part, I used a drill press and some lumber.

You want to support the manifold it is dead vertical, I used a 4x4 some washers and long wood bolts to mount the manifold. You could use the old bolts and the appropriate sized wood block too.

Get some drill bits 1 smaller the hole and one just smaller. Take your time and drill slowly with smallest bit, until you are through if it is a through bolt then repeat with the larger bit,

If it is the one that does not go through, mark your bit length to be a bit shorter than the threads on the new stud, masking tape works well.
Get use the bigger bit and get that bit dead center, then the same process go slow unit you hit the tape, with luck the remains of the stud will just fall away - it did for me.

Now remove the drill bit and chuck up the tap. - YOU ARE DOING THIS BY HAND NO POWER TO THE DRILL PRESS lower the tap and turn the drill chuck by hand take your time. You will get an nice straight clean set of threads.

Put your stud in and put it all back together - took me about 3 hours start to finish - but my bolts came out clean and easy