Need Advice on Professional Car Engine Repair

I’m currently dealing with a serious issue with my vehicle and wanted to get some technical advice regarding car engine repair. Over the past few weeks, the engine has started showing multiple warning signs including knocking noises during acceleration, excessive exhaust smoke, rough idling, and noticeable loss of power. Recently, the check engine light also appeared, and the vehicle has begun overheating after longer drives.After a basic inspection, a local mechanic mentioned that the problem could be related to worn piston rings, low compression, or possible cylinder head damage. They suggested either a full car engine repair, engine rebuild, or complete engine replacement depending on the internal condition of the engine. Since repair costs can vary significantly, I’m trying to understand which option would be the most reliable and cost-effective in the long term.The car is a diesel model with relatively high mileage, but the rest of the vehicle is still in good condition. I’ve also heard that reconditioned engines and rebuilt engines can sometimes be a better alternative than purchasing a brand-new engine.For those who have experience with major car engine repair, what diagnostic checks would you recommend before making a decision? Is compression testing enough, or should I also request a full engine teardown inspection? I’d also appreciate recommendations on trusted car engine repair specialists in the UK who provide warranty-backed services and quality workmanship.