I'd try using a 1/100 Tieren instead.
Zaku would need a bit of work making it boxier and such. Don't really have any ideas on it. But I know the 1/100s are an alright scale for Titans.
PG might be more towards the Warlord/Emperor Titan range, which is good
I don't have any experience with
PGs, but If it has the skeletal structure like the
MG 1/100s do, I'd build up from that rather than the whole thing. Of course nothing stopping you from just using the Zaku as is and using various weapons to represent the titan ones (Heat Hawk = Titan Power Fist thing

)
I supose look at other Titan tutorials for inspiration and you basically have the skeleton (hardest part) done already.
Also scale of 1/100s to marines if you were wondering
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