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in short, I hate the two chimneys of the contemptor dreadnought engine, hoping for a better alternative.
but obviously, No. there is no any third party has made such conversion bits.

So I can only make it by myself ,and it all handmade(because I don't have a 3D printer).

this is an example of removing the chimney and replacing it with a more reasonable exhaust port, and installing some panels to make it look different.
but that's all, just a few minor changes.
here I need to ask for some advice on how to (hand)make a more stand-out or high tech-looking engine for contemptor dreadnought , and in line with the original diesel punk features.

In addition, it has a similar problem how to handmake a DCCW built-in weapon. I replaced the forearm with a 10 mm diameter ABS pipe, so the built-in system have to be redesign.

Ok, so firstly - the removal of the exhausts works fine as per the first pic.
If you want it to look more tech like, you could always file down the panel below the exhausts and add one of the circular type reactors that knights use (and mechanicum)
That should help give it a more refined look.

https://www.games-workshop.com/resources/catalog/product/920x950/99120108015_IKArmigerHelverins03.jpg

The DCCW could also be an easy fix.
You could just add a forearm mounted one like the ferrum (box) style dreads.
Or even make it a shoulder mount.
If you are giving the dreadnought a CC weapon in that hand, just have the gun built in like the custode sword or spear.