5 min to modify. Just put a capped T in the raw water line (pic). One that can have the "in" (female) side of a garden hose attached.
Then cut an old garden hose down to maybe 5 feet, or whatever short size fits your needs, and use a 5 gal bucket full of fresh water or mix with Salt Away etc. Make sure bottom of bucket is higher than the motor ie in the cockpit. Close the raw water seacock. Start the motor. The raw pump can auto siphon / self-prime. Never use a pressurized hose.
I used this method to winterize with anti-freeze, back when the boat and I were in the cold (Annapolis). In So Cal now so don't need.
All said, I've never flushed the raw side, except for winterizing which I haven't done in 4 years, and have had the HX out recently and it's clean. Very minor calcification. No corrosion. Motor has 1,400 hours on it. I've put over 300 on it.
Just keep the HX zinc maintained.