the original source (Civ3) had purple background and shadows which OTTD doesn't know.
Yeah, thats the main problem with trying to convert Civ3 graphics to OpenTTD. Most attempts then dont really "fit" into OpenTTD's style.
These graphics though are rather close to it, so using them as inspiration and for determining the general shape (lenght/width/hight) is ok (that is how you can credit Orthanc then and you wont have copyright issues).
Maybe a way to go is by looking at some of the existing shipping sets, deriving a hull from those (by taking an existing hull and resize/reshape that, more fitting Titanics hull), for example from one of the larger freightships of FISH/SQUID.
For the superstructure you could look at the passengerships of FISH/SQUID or those from the WSF Ferryset (sources can be found in the .zip): viewtopic.php?f=26&t=59499&hilit=ferry+ ... 0#p1108158
Those sets are GPL-licensed, so as long as you also comply with that license, you can freely use/copy their graphics.
Regarding the rigging, start out with just the masts, the wiring may possibly even not be visible in such an OpenTTD-scaled down ship and could always be added later (or in an extra zoomlevel)
Orthancs graphics you can then use to compare your work with, to see if you get close to that.
A very good thing is that you already are able to code, so you can check the graphics ingame yourself, to see if they fit in better.
So, a bit more work than using Civ3 graphics, but probably worth the effort (make it easier for yourself by copying existing stuff from those GPL-licensed sets), I think a lot of people would love to use the Titanic in their games!!
EDIT: To be on the safe side though, I would recommend to remove your first version of the GRF....