If I ordered a custom car engine 6 months ago and was told that the build time would be 10 weeks, If I still haven’t received anything can I request my deposit back?

Yes, you should be able to get your deposit back, so long as you can show that the agreed-upon delivery date was 10 weeks. In that event, the builder has materially breached (violated in an important way) the contract; when one party to a contract violates it in a material way, the other party is excused from its obligations and can seek the return of any deposit. The more documentation you have of the delivery date, the stronger your case (e.g., best would be if you had a signed agreement or quote indicating when you would get the engine). In the absence of any documentation of when the motor should have been delivered, you may wish to send the builder a letter (sent some way you can prove delivery; e.g. Federal Express or certified mail, return receipt requested) which recites the time line (e.g. when you commissioned the engine; when they promised it; how long has gone by), stating that the engine is overdue, and stating that unless they provide it in some reasonable time (e.g. 2 weeks) you will terminate the contract and demand your deposit back. This will help document the expectations and their non-performance.