Not exactly sure how standard mine is as it's been rebuilt in the past. Also I've got an S2 not sure if same as S3? Engine cover has elastic rubber catches either side to hold it to the boot floor. Their is also a hinged access panel which locks with 2 adjustable metal hook type catches into the engine cover.
The boot floor has 2 removable panels either side of the engine that bolt into the chassis. The bolts were missing on mine originally causing the floor to "bang" as I hit undulations in the road - easy fix, new bolts!
Not sure if this helps...........
Mat 79 S2