Has not run in 25 years is critical piece of info...........likely rust and pitting on the cylinders walls and stuck rings and maybe partially stuck valves and rusty valve seats.
It is some want amazing the engine turns over and was not stuck from sitting.
No loss is trying the flush trick but chances of success might be low.
Low budget, you could disassemble, clean things up, replace the rings and the other bare minimums to get it back on the road. Not the best solution but these engines can be forgiving with get by specs. As mentioned used engine might have the same or worse problems. Obviously Best used engines come from running vehicles that are being converted to modern drive trains.
