Garage changed my engine

I gave them my bike in April to fix the gearbox,the bike is a lowrider 350cc they called and send massages to me saying they can't fix it instead they have a secondhand gearbox to replace mine l agreed now they bring my back with a 250cc engine mine was a 350cc they changed my engine without my concern now if l ask them for my original engine they are saying l have to pay them for storage which was not party of the contract from April to July and the bike just drove for 20minutes and it died it's not starting Again they quoted me 9500 to replace my gearbox and they changed my whole engine now they say they did me a favour if and lf l need my old engine l have to pay them which is rediculous already my bike dropped value and l never told them to change the engine so l said they can fix their 250 cc and give back my 350cc engine or else they put back my 350cc engine and refund me my 9500 back