For legal reasons anything I say can't be used seriously I suppose, but....
"I'm fit and only 29" makes you very unlikely to have have an ischaemic heart attack/myocardial infarction, unless you are a cocaine user or have a familial tendency towards high cholesterol. Your mechanism of injury also makes an MI less likely than traumatic cardiac damage.
I imagine what's happened is that someone has requested a cardiac blood test called a 'troponin' which IS elevated in a myocardial infarction, but is also elevated in lots of other 'illnesses' such as heart infections, blood clots on the lung, trauma to the heart etc.
If I were you, and really wanted to clear things up, I wouldn't accept 'heart attack' as a diagnosis because from now on you will always be labelled as having had one, which will seriously affect your life/home insurance etc.
Ask if you really had an MI (which they would then go on and perform an angiogram for), or if the heart trauma (from the crash obviously) would have raised the troponin.
However, i still wouldn't take it all too lightly as it must have been a whack to cause a trop rise, and would take your cardiologist's advice on whether to enter the race or not.......