What is the accepted, standard, enthalpy change value for the reaction between Mg (s) and HCl (aq)?
There are no values that everybody agrees on (except when those values are exactly defined, e.g. zero for elements in their standard states). This is because random error is produced in a different way by each possible technique that we can use to measure values. I can’t find an enthalpy value for MgCl2(aq), and it would not be completely correct (although it would perhaps be a good approximation) to use the values for Mg2+(aq) and Cl-(aq) anywhere online. You really do need the hardcopy sources. Try the CRC Handbook of Chemistry and Physics. It has been the gold standard for several decades.