From the 2012 CARS regulations.....
28.1.2.3 Mandatory equipment
(a) Helmets meeting NRR.11.1.6 or bearing “Snell Memorial Foundation” 1995 M or 1995 SA (or
more recent) labels must be worn by all drivers and passengers.

11.1.6 Only helmets meeting one of the following standards will be accepted for competition in any performance
rally and must be worn by all competitors when travelling in special stages:
(a) FIA Homologation -FIA 8860-2004
(b) Snell Foundation -SA 2005
-SAH 2010
-SA 2010
(c) British Standards Institution -BS 6658-85 type A/FR including all amendments
(Standard expires December 31st. 2013)
(d) SFI Foundation Inc. -SFI spec 31.1A and SFI spec 31.2A
-SFI spec 31.1/2005

It is strongly recommended that helmets be discarded after five years, due to deterioration, or after a serious accident, which could weaken the helmet.

Long story short.....You're helmet is technically acceptable, but note the recommendation around age.
If you decide you like rallycross and will continue to participate you can get new, inexpensive Snell-rated helmets fairly inexpensively.
It's false economy to scrimp on safety equipment.