We can find the masses of the isotopes from various online resources (e.g. Wikipedia has a list of the properties of isotopes of all elements). The masses are:
Po-210: 209.9828737 u
Pb-206: 205.9744653 u
He-4: 4.00260325415 u
Here u is the atomic mass unit:
1 u = 1.66053904*10^(-27) kg
and the isotope masses refer to the atomic masses, this thus includes the electrons and the binding energy of the electrons.
When Po-210 decays by emitting an alpha particle (i.e. a He-4 nucleus), we end up with a fast moving He-4 nucleus and a doubly ionized Pb-206 atom. If we ignore the change in the binding energy due to re-arranging the electrons, then we can also shift two electrons from the doubly ionized Pb-206 to the He-4 and simply ...
I explain how to take recoil effects into account to calculate the energy of the alpha particle.