COVID-19 and myocarditis

A couple of excerpts.

the extent of myocarditis was not related to the severity of the initial illness or overall course of the illness

That said, even if just a small percentage of Covid-19 cases lead to cardiac complications, the pandemic will elevate the risk for millions of people who regularly exercise. As an example, the study indicated that patients with only mild Covid-19 symptoms developed myocarditis as often as those who were hospitalized, elevating the chance that those who are unaware they have Covid-19 could also be at risk.

This is relevant because athletes with myocarditis must stop all strenuous activity and workouts for many weeks, if not months, until the the inflammation in the heart resolves. The risks of not curtailing activities can result in dangerous arrhythmias, heart failure and death (cardiac arrest).

I'm left with the following questions.
- Does it take an MRI to identify mycarditis?
- Is the risk of death related to the severity of symptoms of myocarditis?
- How common are heart abnormalities in the age of the population they tested? In other words, what % of the issues are likely caused by the virus.
- How common is myocarditis in the normal population? I assume it's low, but it's also unstated.