[400TT]

I would expect that without performing emissions tests you engineer will want you to present an engin that is known to pass ADR 37/01 emissions requirements.

ADR37/01 compliant engines were phased into Aus between 1997-1999.

ADR37/01 compliant engines can also be sourced from the states. 1990 onwards is easily considered compliant, although people have justified older USA engines.

So I expect you are going to need to present what appears to be a completely stock ADR37/01 compliant Aussie or USA sourced engine to comply with the regulations.

ADR37/01 had no emissions tests for LPG, so you can do anything as long as you run it on LPG. So that's why the (now buried) LPG option worked for you so well.