Are there any rules for this my group has run into one and want to claim it but i have no idea how to build it, though just for reference its about 50km long and 15 km at the widest though as a note it isnt a space hulk or anything

Honestly even if it isn't a hulk I'd say that no-one currently living would have the required knowledge to crew such a ship and the best option would be to trade it's location to someone better equipped to deal with it like the Adeptus Mechanicus.

Then you get all the kudos and they take all the risks.

while that would indeed be the logical thing to do they dont wanna do that, and i dont wanna railroad them into having to do that

With something like that.. it's very much a... great you have it.. but can you keep it? The scale, size and rariry of the ship. In theme, The AdMech would do whatever they could to reclaim it, as it is most likely filled with rare and lost tech. The Traitors would do anything to take it, as it is a great weapon... and since there where so very very few of them (pretty much just one per Space Marine Chapter) It's original Owners would sacrifice much to get it back.

It's one of those things that, per theme, there is very little to justify them keeping it. I doubt they could "Claim it" let alone crew it and get it operational agian. They took the efforts of an entire forge world to build and maintain.. each. So If the players don't have the ability to refurbish it and get it operational.. and maintain it... is hard, very hard to keep.. and I doubt any faction would let them claim it without great contest.

Further more, The Imperium looks very closely at the few Rogue Traders who have Battle Ships... I doubt they would stand for a RT owning a ship that.. and I quote "Has the Ability to battle entire fleets on its own" Its a far to dangerous weapon to get in the hands of a RT and, honestly, it is wasted on a RT. the Imperial Navy would do whatever it is to get it, as it by itself is like adding another fleet...

So to recap:

Super ship, 25 KM in lenght, with overlapping void shields, and enough fire power to take on an entire fleet

Only a handful where ever made, and pretty much all went to Space Marine Chapters.. I doubt the SM will like one of their prized relics in the hands of a slimy RT

Nearly all of them where destroyed in M31

Super Rare Tech.. ADmech Magnet..

Nearly impossible to operate without the admech backing you up

Nearly impossible to maintain without the admech (and I doubt they'll want to help.. they'll want to take it)

So I am not saying not to give it to the players.. I am saying not to make it any where near easy. There is a ton of theme stuff stacked against them and the logistics of it are a nightmare. Don't tell them no.. but make them work for it, overcome the challenges and fight to hold it from the people and groups that want to take it from them. You have a few entire adventures right there.

I completely agree, games don't work if you tell players no very often. What they are undertaking is an endeavor. And likely a Greater one on scale of colonizing a system with no established warp routes. Make them understand just how unique it is, the manpower and resources involved, and the overwhelming difficulty in keeping it anything but a floating hulk. Make them earn it.

Give it the stats of a Ramilies Star Fort?

Or a Blackstone Fortress?

wait blackstone forts have stats?

Not that I know of, no. Back in the day, people here would occasionally discuss the similarities between, and formula for how you could take a BFG stat block, and convert it to RT (not a perfect science, by any stretch of the imagination), and I asked if anyone would, but got no takers; not a big surprise, when they aren't in this part of the galaxy, or something any player could have, but I hoped. No, I just mentioned it in comparison to the Ramilles which, strangely enough, had bigger, more complicated stats, back in the BFG days.

while that would indeed be the logical thing to do they dont wanna do that, and i dont wanna railroad them into having to do that

The alternative being the players railroading the GM into permitting the salvage of a battleship the Ad Mech no longer has the capability to build, repair, or maintain. This will be followed by player mayhem as they roam in search of another fleet to destroy. You'll need to develop some rules that streamline the adjudication of this ship destroying squadrons of warships.

You gave them the Holy Grail. What can you offer them next time? If they insist on restoring the ship the rest of the campaign should likely revolve around the party visiting major shipyards around the galaxy looking for spare parts, specialized labor, forbidden knowledge, and maybe a tour of the last working model.

I actually ended up telling them that it belonged to vulkan and they decided on there own to bring it to nocturne! and basicly gave it to the salamanders for free ( though they did get a full merchant fleet from them because of it i felt i had to reward them for that)