Cylinder Head Type:
   There are three basic types of cylinder heads used on Subaru engines:

1990 - 1999 SOHC
   These heads are the most compact and were used on all non turbocharged four cylinder engines up to 1999 except the 2.5. Engines with these heads are all of a ‘non interference design, so no damage is done if the cam belt breaks.

1999 --> SOHC
   The original SOHC heads were replaced for the 2000 model year with a new design which flow better and feature roller cam followers. The ports look very similar to the DOHC heads. The following link shows just what good heads these are - they blow away a Chevrolet LS1 V8 on the flow bench 99--> SOHC vs Chevy LS1 V8. Apparently some engines with these heads are of an interference design, so if the cam belt breaks, the least damage to expect it for the valves to bend due to hitting the pistons.
   The 99 --> SOHC heads are more bulky than the earlier SOHC heads, but the critical bit in terms of getting them into Beetle bodywork (the size in the area of the cam drive) is the same.

DOHC
   The basic design of the DOHC heads is basically the same from the 1990 Legacy Turbo up to all but the latest STi’s. Some (mostly STi’s in the UK) feature Subaru variable inlet valve timing system, which is basically a bolt on addition to the earlier head design. It is proportionally variable, rather than just switching the inlet cam between two positions like many lesser systems. All engines with DOHC heads are an interference design, so if the cam belt breaks, the least damage to expect it for the valves to bend due to hitting the pistons.
   The DOHC heads are fitted to almost all turbo engines, and the 1996 - 99 2.5 naturally aspirated engines. They will be much harder to fit into a Beetle body compared to the SOHC designs.

ADD PHOTOS OF HEADS