Custom molding help

Anyone got any tips / recommendations for epoxy / general molding putty for adding details / parts to a transformer? Specifically I want to turn Titans return Hardhead into gurren lagann (new chest, drill Instead of guns, etc)
SEIBERTRON - Your Planet For Everything Transformers
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https://www.seibertron.com/energonpub/viewtopic.php?f=143&t=107421