Work not completed and possession not give till date of flat

Respected Sir/Madam,

We have booked a flat in Raigad dist, maharashtra . As per registered sale deed the completion and possession date was Dec 2013. However till date it is not given. Internal work of our flat, water supply tank & electricity meter not done, outside drainage system is incomplete. This building is not registered under Maharera. Builder is saying that he got a certificate from Grampanchayat for completion hence rera registration is not required.

We are following with him for last 3 to 4 years via emails, letters with Registered AD, phone calls, messages etc. He is giving reason that he does not have funds and the market is down. He says that he will do the work once he gets funds from banks. He has also offered to complete internal work and outside drainage work at our cost (reimbursable basis) and he will give possession. 

Kindly advice what course of legal action we should follow to recover our money with interest or should we complete the rest of the internal work of our flat at our cost and get possession?. We have already paid 90% of flat cost to the builder as per registered sale deed.