I don't know what thread it is - but, it's a pretty safe bet that its some sort of metric thread.
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Before you proceed, I -highly- recommend you spend an hour or so browsing through Fastenal's Technical Reference; at the very beginning, it has a lot of great information about screws.
https://www.fastenal.com/content/doc...renceGuide.pdf
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Once you've had an opportunity to review the screw reference, Fastenal also has a recommended torque calculator which will answer the question "What is the hardest I can torque this screw/bolt down?" Keep in mind that this calculator only applies to the screw/bolt;
you may not be able to go that hard if, for instance, the head has aluminum threads without any kind of steel inserts.
https://www.fastenal.com/en/83/torque-calculator
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"Take one of the stud bolts down to the local hardware store and identify the diameter/thread pitch. Get a pair of replacement studs. Get the appropriate heli-coil kit, including the correct drill bit and tap" is what I would say, except for the fact that you've already drilled one of the holes out and tapped it to an English thread size.
At this point, you've almost certainly screwed the pooch. Unless the drill size for the heli-coil is the same/larger than the OD of the 1/4" tap that you used, the only way out now is to replace the head.
If the frame is in the way, you really have no choice but to drop the engine out, unless its a vertical engine. Also, make sure that you are using a torque wrench to install the cap nuts; on my Xciting 250, which uses M8 stud bolts, the service manual says to torque them down to 14 ft-lbs (this compares with the calculator, which states that for a dry coarse M8 grade 8.8 stainless steel screw permits a maximum of 18.8 lbs, derated to 80% = 15 ft-lbs). Also, make sure to use anti-seize compound on the studs. I would suggest tightening the studs into the head first, by putting two nuts on them, tightening them together, and driving it in with the outer nut. After you can use a wrench on the inner/outer nut to separate them and put the exhaust on. Do this on the bench before putting it back on.