Please I need help on this matter. Anyone with good advice is welcome.

I work for an organisation as a salesman.
In 2012, the company gave me an official car to carry out my official duties with a policy that I will have to pay for the car in instalments for 4 years but the car must be used for their job throughout this period with a written agreement.
According to the agreement, they are responsible for the vehicle maintenance. This was adhered to by both parties.
In 2016, I concluded payment for the car but the company refused to transfer ownership to me explaining that I should be patient for them to buy a new car for me before transferring ownership. I continued my job (with the same vehicle) based on their explanation but no replacement vehicle was bought. We did not sign any other agreement on this.
Last year the car started smoking and I complained to the management to change the engine of the car.
They refused, and last month of February, they sent me a letter transferring ownership to me.
I replied with a letter refusing to accept the offer but they refused to reply. I am seeing my employer using its advantage on me because I'm on its payroll.
However, I want to go to court to claim all the money I paid on the car even with interest; but will like to know if my case is valid looking at the fact that the agreement is already more than 6 years old.
Kindly advise me if I need to get a lawyer or just accept my fate.
Thank you.