Hey bud I'm in Livonia !!! The web site is:

http://www.sterndrive.cc/home.htm

They sell Alpha 1 Gen I and Gen II. I'm sure China made, but OEM MErCruiser was Mexico to begin with. I have not used them, but have not heard anything bad yet. Uppers or lowers $600 each completely new with 1 year unconditional warranty.

Not sure when Gen II started, but they sell them also. They are called SEI and all parts supposedly interchange with Merc parts and tools. I'd try them if needed.
Does your boat have twins?

I think the old drive off of the 250 Nova is called a I(one)-drive not alpha, it was older. The gear ratio would be different on a 4.3 or smaller engine.
Plenty of rebuilt or replacements for your Alpha out there.

what power is in the nova this is important for two reasons gear ratio as well as the length of the yoke in some cases. also there is a big model break in 1983 my nuber is 1-419-732-1777 and i have hundreds of alpha ones new, rebiult, used. as well as all components

these drives are actually made in thialand (housings ect) and the gears and shafts are glm. they are assembled in clearwater florida at their parent companys site. they are owned by sbt a personal watercraft engine remanufacturer. i have repaired and experimented with a few they have threir problems, but as the saying goes you get what you pay for i would rather put 1300 in a drive after everything is said and done than skies the limit with mercury. i dont carry these drives because i can build a reman for same money and i have never been a fan of glm gears but the people at sterndrive engineering are nice enough and a welcome presence to force mercury to bring their pricing down

Just to mix it up a bit! I just got done putting a gen 2 alpha on a gen 1 gimble. Had to block off the drive lube hole-extend the shift fork, (local welder did this for 40.00) And grind down the inside of the gimble ring fingers that guide the drive. And It's working great. All this on a 84 searay seville.

"old 250 Nova" isn't a very descriptive term.

What year Nova and what power? That will help us determine what is likely hanging off the boat.

And has been mentioned already, ALL "Alpha style" drives (including the Type I) will more or less bolt up to the same gimbal with inexpensive mods (clearancing, custom gaskets, shifter extensions, swapping out input yokes, etc.)