Someone who knows lots about cars?

So my engine has been working fine, i have a oil leak that can be maintained by stoppers and more oil, but now my engine sounds "weak" and makes a high pitched whirring noise. The engine is also sounding a bit eradicate. Kinda relatable to a heart defect or something. This happened so suddenly without much warning.
I have a 98 Ford Ranger with a remade V4 engine in it.
I know nothing about cars and hope someone can help me at least a little, and maybe tell me what it might cost if you know the possible problem.
Thank you

Added (1). Stoppers. They are thicker oil or something to help stop leaks.

Added (2). And also, by remade engine, i meant refurbished. I just couldn't remember the word for it earlier.

What are stoppers?

"They are thicker oil or something to help stop leaks. "

Then they have ruined your engine as the engine is designed to work on a certain viscosity and you have changed it so it will not be doing it's job. You may need a new engine now when you could have just fixed the leak.

Don't panic, you probably have a spark plug gone faulty or a spark plug lead, check these first, to check, use an insulated glove and with the engine running on idle, remove spark plug lead one by one, then replacing back before removing the next one, when you hear no difference to the engine's performance that is the actual plug or lead that is faulty, plugs can be checked by removing the plug and with lead still attatched grounding plug metal to cylinder head metal and engine on idle, be aware of fuel that could ignite nearby.