I would like you to kindly suggest how I should pursue this and whether my case is strong and Good to get a favorable verdict.
Your father did a mistake by not getting the registered sale deed on his name during the time of purchase, instead he made a power of attorney deed.
The power of attorney deed was no doubt valid but an unregistered sale agreement is not at all valid. Further neither he nor you bothered to execute a proper sale deed in your favor hence the sale agreement is very badly barred by limitation at this stage.
Legally speaking, an unregistered sale deed itself is not valid, so the question about an unregistered sale agreement does not arise at all.
So mistakes after mistakes are found in your side.
The original principal would have cancelled the power of attorney deed in your father's name and might have sold the property to a third party and the sale transaction cannot be said to be illegal or invalid.
You have not mentioned that under what relief you have approached court?
So, wait for the outcome of the court verdict.