On the Harley RI, I guess it's not really practical. If they were to reproduce classics, the price would be so far out there no one could afford them.
On the re-issue and practicality, yes and no. If you are mechanically inclined, and have a workbench and shop space (that will no longer be available for anything else), you can duplicate a very nice old V-twin. Though I've been out of touch with the European sources for a while, I see that some are still active. Quality reproduction parts (read that as better than original specs) may still come out of Poland. Of course, prices of parts are such that there's no real big savings, except that your labor is free. That's the killer expense.
The little WR racer can be somewhat duplicated, though I would advise anyone attempting such a build to not plan to burn the roads up. They're tough, and will outlast most people. Ride them on the backroads, and chains, tires, and oil will be the primary maintenance. Enjoy the ride, and occasionally wind it up on a straight stretch. The oil pumps in the 45's are vane pumps, and cannot pump against too much back pressure. So, no filters. However, it's very easy to make a small filter press. One that you can attach to the oil drain on the tank, use the pump on the press to circulate the oil through a really good, high filtration filter, and back into the tank. Just let it circulate for a couple hours, and you're ready for the road, again.
Your can buy all new parts:
http://webshop.45flatheadservice.com/epages/63114975.sf/en_GB/?ObjectPath=/Shops/63114975/Categoriesor you can do as I did for years, and take in the many swapmeets that pop up. Parking lot swapmeets were the best. You would rent a parking space (if you were selling, like I did often), park your truck, and sell out of the back. Most of these swapmeets were run by 'clubs', and policed them better than you would ever imagine. Nothing was stolen.
Anyway, if you are fortunate to buy a set of cases with title, you are way ahead of anyone else. But, a notarized bill of sale will get you a reconstructed title.
And because it's your build, put whatever you want on it for lights, seats, whatever. That way, it's a classic 40's and 50's Bobber. Most builds like this take a year or more, so that the wallet isn't put under too much strain.
Jack