I have quite a few Magnum engines and the 52 is one of my favorites, at this time
I have 3 of them, the last was extremely tight when I received it and like you had
a heck of a time getting it to spin over with the plug seated. The procedure I used
to disconnect the fuel line and inject some after run oil into the cylinder without the
plug in place and spin it over for periods of 15-20 seconds at a time, maybe 4 to 5
times then reconnect the fuel line, prime the engine by placing finger over the carb
and then reinstalling the glow plug but do not seat it, leaving it a couple threads up
from seating the copper washer, this has the effect of reducing the compression in
the combustion chamber and should make it easier to turn the engine over while trying
to start it, an old model boat engine trick with tight ABC engines, once the engine starts
tighten the glow plug quickly and the engine should continue to run. Keep it a little rich
but not slobbering rich, as that can damage to the con rod. Run at least 3 full tanks of
fuel through the engine before going airborne and on the first few flights don't throttle down
too far out as the engine "will die" until it is totally broken in. Magnum engines are great
engines for the money and you will be rewarded with long engine life if taken care of.

BTW I normally use the OS #6 plug on all of my glow engines with 15% fuel.

Bill D.