For this to work as a starter, I'd need it to generate as much power as
a typical starter.  During cranking, I believe a typical starter will
draw 200A at around 11 volts.  I'm sure someone will correct me if that
isn't typical.  That's 2200 watts, or around 3HP.  One way to go would
be to adapt one of "hub motors", but the largest of the flat motors that
I've seen are rated at only 500W (<1Hp).
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I'm going from memory here, but 2-3 years ago I did some kind of measurement
on the starter current (don't even remember what I did, but I think the
initial surge of current was way up there, but once spinning I was surprised
that it was down around 60 -80 amps.  This was during initial engine tests,
and perhaps the compression was low.

FWIW,

Al