On the MY99 revised inlet manifold that is not compatible with earlier WRX's.
Camshafts were a revised design that won't fit earlier heads.
Steel type head gaskets came standard on MY99.
Larger Cambelt tensioner idler pulley and a revised cam belt adjuster.

Thats all I can find...

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"You can turn your back on a person, but, never turn your back on a drug. Especially when it's waving a razor-sharp hunting knife in your eye." - Raoul Duke