Generally speaking in America it’s very difficult to obtain custody of a corpse after the authorities have it in their possession. While there are many types of funeral options viking burials are not one. Regular fire does not burn hot enough to fully burn a human body, which is why cremation uses special equipment. Generally nobody wants partially burnt corpses in their river, so you’re either going to be illegally polluting or going to international waters. Generally you’ll need to convince some combination of funeral director, medical examiner, coroner, hospital director, and the police to give you custody of the body. It seems unbelievable to me that you’ll be able to convince all those people to give you a body for a viking funeral

On to illegal ways you could accomplish this: not reporting a dead body is not only a crime but also makes you incredibly suspicious for fraud at a minimum and murder at the max. Not acquiring a death certificate will make dealing with any estate very difficult. Stealing the body from wherever it is located will likely get you charges for defacing a dead body. You could bribe some people to look the other way but if that fails that’s also a crime

In short, no you can’t really do it. However, this is America, so presumably with enough money to the right people you could probably work it out. But we’re talking .01% level money and connections