This mission has 4 varieties - basic, HART, Foam, and HART and Foam.
The basic and foam still require an SRV to attend, which is a HART only vehicle. This mission can then not be completed if you don't have HART, but will still spawn.
Scrapyard Fire Large - Basic, required SRV from HART
Scrapyard Fire Large - HART, requires PRV, SRV and welfare from HART
Scrapyard Fire Large - Foam, requires SRV from HART
Scrapyard Fire Large - HART and Foam, requires PRV, SRV and welfare from HART
I think the SRV requirement just needs to be removed from the basic and the Foam variety.