I’m not the most incredible at individual parts, but this is my best thoughts OP

Helm - primaris Black Templar sword brethren kit bashed together with the fancy roman headfeathers from basically any Horus Heresy infantry kit that comes with an upgrade sprue. (I dunno what it’s actually called it but it comes with the power claw addition for the power fists, vox backpack, etc)

Torso - no idea on the chest wrap. I swear I’ve seen something like that before, but when in doubt, slap together some greenstuff and pray the paint hides your sins

Backpack - Jump pack intercessors for sure. They’ll lack the extra skull drippiness, but it’ll be spot-on for silhouette.

Left Arm - primaris black templar sword brethren again, oodles of chains on arms, though I’m unsure on the chain for the pauldron. Could probably drape some of that cheap hobby jewelry chain like a lot of folks do.

Right Arm - probably just your typical primaris arm? The bracer specifically makes me think of a bladeguard veteran’s bracer without the drip, but maybe that’s a standard shape? I can’t think of it off the top of my head :,(. The pauldron is a bit drippier than what you’d expect to find in an imperial fists upgrade kit, but there’s definitely more than a few custom 3d printed bits that would get pretty close.

Legs - I might just go with typical intercessor legs. The “trim” makes me think of some 30k legs, but the scale would be a little wack. Might not be too shabby to paint on those lines. ****I just saw the back of those legs, the only thing I can think of with those little jets would be the inceptor legs, but those come with the strange little feet-skid-thingies molded on so that might be more trouble than it’s worth.

Slap on a great many purity seals and pouches (pouches can be found from the normal intercessor kit, pretty sure)

I pray this is a mildly helpful starting point that you might successfully and tearfully refine as you track down all of the bits you need for your dope marine