Mazda Repair: 1990 miata crankshaft pulley keyway and boss, crankshaft pulley, proof design

Question
Can this be repaired in the car, or do you need to teardown the motor?

Answer
Not properly.  Some shade tree techs have accomplished short term fixes however, the only proper course is to remove the engine.  The crankshaft and oil pump can be upgraded to a 1991-1993 design.  In most cases the crank and pump can be dropped right in without a full overhaul.  It depends on mileage and internal wear however.  The parts are still available new from Mazda just pricey (crank ($350, pump $180).  Another alternative is to locate a good used 1.6 though they are hard to come by nowadays.  Collectors, restorers and race people have scooped them up in numbers.  This issue can be overcome, it will cost you, the flip side is these engines and cars will still be here 10-15 years from now, they are that well built. (except for your crankshaft design which ironically is not as much a design flaw as a technician error with a NOT monkey proof design, I have dozens of 1990's still coming through here with no failure showing miles over 250k)