It's all in the CARS rulebook, however here's the basics:

Open - Chassis and motor have to come from the same manufacturer, 2.5L max displacement, 4wd anything goes pretty much, except for no sequential gearboxes and no electronic controls for suspension or diffs other than the centre. 34mm restrictor for forced induction. Subarus, EVOs, etc.

Group N - Essentially a production class, with parts homologated by the manufacturer for use on the specific vehicle permitted/required. You've gotta have homologation papers for the vehicle in question, and have very strict limits on the parts/prep. Subarus, EVOs.

Group 5 - 2wd open class, essentially, with a 5.1L displacement limit for NA motors or 3L for forced induction. As long as the motor and chassis are from the same manufacturer and it's 2wd, anything goes. No restrictors. Mustangs, Volvos, Nissan 240s, Porsche, etc.

Group 2 - 2wd open class but no forced induction is allowed and the adjusted displacement limit is 2.4L. VWs, umm, more VWs...

Production - P1 for up to 1.4L adjusted displacement, P2 for up to 1.6L, P3 for up to 2.0L, and P4 for above 2.0L. In these classes there are limits on what you can do to a car such as the motor has to be stock, can't be lighter than the listed curb weight, etc. 32mm restrictor for turbos. Everything from Justys and Swifts in P1 to Paseos and Civics in P2 to Golfs and Neons in P3 to Subarus in P4.

The multiplier for pushrod engines only applies to group2, and group5.