hey amigo, I know what it's like trying to figure out a custom. so here's my 2 pounds worth:
OPTION 1:
for a body you might want to look at a true type narrow shoulder (here's the link)
Custom Made 1 6 Scale Figure Body "Narrow Shoulder" Fit 12in Headsculpt 15 | eBay
then grab lycra superhero suit (all this can be found on ebay or Tony Meis makes the best check out his stuff).
To add muscle underneath the lycra suit use a 2 part apoxie putty that air drys and just go over the muscles you wanna add bulk to.
heat the vinyl parts you wanna use by either boiling water or a heat gun/hairdryer :remember heat gentle so you have more control over the piece rather than melt it and your hands at the same time lol.
use a few dabs of glue when you need to add the vinyl parts to make them stick to the lycra under suit
OPTION 2: glue the pieces to a cheap body like a DID dragon ,you might need to cut and stick to get correct height proportions (once again ebay will find you cheap ones)
next use apoxy putty to sculpt in the missing parts, REMEMBER to add the articulation points or the figure will become a mummy when the apoxy hardens.
hope this helps ,im beginning work on my own version of BATMAN cross between arkham knight and my vision ,ill be putting up wip pics of the whole build .sum might help you ,with ideas or future customising.
hope that helps buddy.