ASP and Magnum are both manufactured by Sanye. They appear to be identical but I do not know if there is a 100% parts interchange on every model.
One reason Magnum costs more (in the US) is because Global Hobby provides warranty support for them.
I suspect there is no warranty on ASP engines in the US but that is speculation on my part.
If the price is good enough, lack of warranty may not matter.
My brother had two Magnum 91 four stroke crankshafts break in one season. Global repaired both engines for free.
I do own an ASP 61 two stroke and it has been a very good engine.