Nasty engine ( didn't change oil often enough)

I don't think you want to do that it may dislodge a chunk of crap . I have one like that , the engine is " sludged up " The oil gets dark soon after an oil change . I just wait till it gets darker than usual . Or you can do them every week if you want .

I can do an oil change every week? Is there an oil or can I add Techron or something to slowly clean it over time. I am just trying to avoid replacing the engine. So I hope I can nurse it back to health over time.

Dislodging too much of that crud at once can plug the oil pickup screen at the oil pump. That will cause oil starvation which will ruin the engine. I'd leave it alone and do regular oil changes until it dies or you get rid of it.

How regular should the oil changes be? Will once a month be of any benefit? So you dont reccomend any additives then?

I would change the oil more often than you have been doing. Perhaps every 2,000 miles.