Car purchase private engine damage concealed

Hello! I bought a Smart from Private 2 months ago. The day before yesterday the engine suddenly started to tick, the car did not drive faster than 60 kmh. Go to Smart with the car. Problem described, then the good man entered the vehicle data into his PC and found it. On 18 December the previous owner was in the same Smart Center with similar problem, the car was completely checked there. Diagnosis: Turbocharger torn, consequence engine damage, as well as various other defects. The damage was caused by On December 21, the seller was at the Dekra, had the car presented and examined there. The damage that Smart found was not confirmed there. Dekra Siegel was given. Then we arrived, looked at the car, looks good, all reports, except on the one of the Smart dealer!, were finely stapled. Sounds like a good offer. Purchased for 2600 euros. From the examination and diagnosis of the Smart H We then had the car checked again at Smart, maybe the damage was repaired in another workshop. However, this was clearly not the case. Summary: Car bought from privately, seller has not kept a good deal of information, sellers only hesitated to report, will probably be slandered by the friend in the future. Based on the statement of the Dekra seal. Does anyone know if the Dekra can even look into the engine at all has someone experienced something similar, or has someone got tips on what I should do now, since I think I have a right to withdraw from the contract, or seller has the car repaired, or I sell the car and the seller gives me the remaining sum…?? What should I do? I have legal protection insurance after rolling my documents stupidly! unfortunately not.