24" MTB Crankset Truvativ E300 3.0, 42/32/22 Right arm replacement?

Fitting a helicoil or thread-replacement is the correct fix, or the whole crankset needs replacing. From your photo, the right-side crank arm and spider are one part, and it may be rivetted onto the chainrings.

Sadly, thanks to some poor historical choices, the thread of a pedal is a weird combination of 9/16" diameter, and 20 TPI, where the common industry standard is 18 TPI. Thus, the taps and coils are specialist to the bicycle industry and are therefore very expensive. And additionally, the left-side pedal is left-hand thread, so the tap and inserts are all duplicated.

I'd bet that your tool library tap set won't have the required items unless its a specialist bike set.

Even having the 20TPI parts isn't enough - making the pedal thread bore at right angles to the bike is also critical, and the accuracy required is too much for a bench vise and a hand drill. You'd need a vertical mill ideally, or at the very least a rigid pillar drill/drill press.

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Your cheapest option might be to look for a donor bike.

Some more info: 42/32/22 refers to the gear tooth counts of the 3 chainrings that came with the crankset. 165 does indeed refer to the crankarm length, measured from crank (bottom bracket) axis to pedal axis. 165mm is on the shorter side, which makes sense since your bike has 24" wheels rather than 26" or larger wheels.

When replacing crankarms, be sure that the left and right have the same length. On the Truvativ E300 3.0 crankset (circa 2011?) it seems that some use a square taper bottom bracket and some use a Truvativ-specific "isospline" bottom bracket.

There are plenty of instructions online for removing and installing crankarms. Your crankset is a "3 piece" (2 crankarms and a bottom bracket); high-quality instructions for removal and installation are found here at parktool.com among other sites.