Yes it is possible to rid a boat of foul odor.
First, I would recommend reading Peggy Hall’s book; it is well worth the expense & personal time to know what you need to do & products that will help.
I have cleaned two boats over the years with good results. First, try to determine the predominant odor source, then remove every item from the cabin that has the potential for absorbing & retaining odors. Attack the odor source & eliminate by cleaning, repairing, or replacement. Cushions aren’t necessarily a primary problem source; however, they absorb malodors & become a secondary odor emitter. You can replace the foam as
@dlochner did or if the foam is in good shape, clean it. My foam was five years old so I opted to clean. I mixed a concoction of upholstery cleaner, Dawn, & bleach in a kiddie pool and thoroughly cleaned & rinsed them multiple times until the odor was gone. (same with covers, but no bleach). I used a shop vac to remove excess water & allowed them to remain outdoors in bright sun to thoroughly dry. Then, they were moved into my garage for a couple weeks with an air blower continuously directed on them. It was quite a bit of work; however, the odor was completely eliminated. After the primary odor source is eliminated, ALL hard & soft surfaces in the cabin need to be wiped down with a mold cleaner & disinfectant, then ventilate the cabin with an air blower. After there is no odor onboard, then move the cushions, PFD’s, & remaining items back on board. The head is always a potential odor source, so keep the holding tank pumped out & rinsed & replace the hoses when they become permeated. Try to keep the cabin ventilated to reduce odors. I agree with others that it would be very difficult, if not impossible, to find someone who would do a thorough cleanup that this task demands.
