The engine flush tube has a 'garden' like thread or hose barb. The thread is similar to regular NPT fittings but not exactly identical. Hose fittings don't the same taper as pipe fittings do.
In addition, pipe fittings don't have the smooth rounded over end edge and therefore the rubber washers may not seat correctly or actually can be cut.
One approach to plumbing the flush port is to get a male garden barb hose to garden hose repair kit. They come in male, female or hose to hose. You may find a male hose end piece that can has a brass barbed end and uses a hose clamp. That will work with any type of tubing assuming the ID is correct.
As I mentioned, I prefer plastic over metal since eventually the metal will corrode. If the connections are outside of the cowling, you will see a leak but if the tubing is routed under the cowling and leaks, it can cause a lot of motor corrosion since you may not know there is a leak. Another option is to remove the hose fitting on the flush tube and couple another hose piece or even remove the original flush hose and run new tubing all the way.