New motor runs good but prop not spinning

[CBoSouth]

Sorry if this has been covered but I had no luck w the search. I just bought my first boat last weekend and I started it today for the first time but the prop would not spin. It's supposedly a 90 horse 1971 Johnson and I say supposedly because it has a 55 horse cover on it and I haven't seen any 90 horse 1971's on the Internet yet. I kno it's hydro electric and I filled it w type c oil but when I put it in gear I got nothing. The seller replied "it can't be" when I told him I thought the lower unit was shot so can anyone tell me what else it could be or if I can check the shift linkage somehow? Thanks

[boats.net]

You can check to see if voltage going shift solenoids but parts for those gearcases are no longer available.