Unless the rest of the tractor is beat to hell, you could swap in a diesel mill, and that 4000 would do just about everything you could want. My dad did that with his '68-ish 4000, installing the 3-cylinder diesel out of an early '90s New Holland backhoe. It needs an overhaul now (it has a leaky seal that pees oil, which is no bueno in hay), but he got about 25 years of hard work out of it.