Enya 90-4C worth?
Its very difficult to tell the worth of a larger glow engine anymore. I looked at eBay and saw a New-In-Box (NIB) Enya .90-4C listed for $420. I seriously doubt they will ever get near that much. Even though the Enya .90-4C is still a really great engine, it would be competing against 15cc (.90) gasoline two stroke engines that sell new for $250. I saw NIB Enya .90-4C engines actually selling on RC Universe and RC Groups for $125-$150 about two to three years ago. The fact that it is NIB helps, assuming of course that it still looks show-room perfect, no rust, no corrosion, turns over smoothly, good bearings, etc. Enya is known for making very long lasting and great running engines. An Enya will always be preferred over a similar size Magnum engine. I still fly an Enya .53-4C that is 27 years old, and an Enya R120-4C that is 35 years old. Both still run perfectly.
The price of glow fuel at $25 to $30 per gallon has caused many modelers to convert to electric or gasoline engines when considering engines larger than a .46 size 2C or a .65 size 4-C. Therefore, the resale market for larger glow engines is relatively low and slow. I know several modelers who have actually given away all of their larger glow engines rather than try to sell them.
That being said, it is only worth what you can get for it. How you market it can make a difference. If your dad has a lot of matched items such as NIB airplane kits and matching size engines, you might do better selling items as bundle deals at one of the larger model swap shows. For example, I have a NIB Enya R-120-4C and a relatively rare NIB Carl Goldberg Bucker Jungmann biplane that I got at an estate sale. Each is probably worth around $300. The plane would sell quickly as it is still in high demand and is no longer produced, but the engine not so much. Use a kit to help sell the engine. If I ever do decide to sell it rather than build it, I would probably take the engine and kit to a swap meet and list the pair at $600 and hope to get $500 so as not to be stuck with an engine or plane that didn't sell. If your dad had something like a NIB Great Planes Ultra Sport .60 kit, which would be a perfect match for the Enya .90-4-C, then you could bundle the two for $450 in order to help sell the engine.
If you try a straight sale on eBay, I would suggest listing the Enya .90-4C at $250 and hope to get $175.
Good luck!