David, the first question is why you want to discard the original engine? There are modern engines that are at least as good, but there are also plenty that are not nearly as good - and they are not the original engine, which itself is an argument against putting them on a collectible mower.
The fuel tank placement is just a question of whether you have gravity fuel feed or a suction-type carburetor. I think only Briggs used the suction carburetors, and they used them for their smaller engines, less than 10 hp. In recent years they have been switching the small engines to gravity feed. The relative merits of the two are probably in the eye of the beholder. Gravity feed is simple and faults are easy to diagnose. Suction feed is safer - fuel cannot feed unless the engine is running, so there is no fuel tap, or indeed any place to put one.
A new engine in the 3-5 hp class that you buy now will be made in China, and there are indications that Chinese producers are still on a fairly steep learning curve with regard to production quality. You can read about some cases on this site if you do a search.
The choice of what engine to use is of course up to you. If you replace the original one you will reduce the value of your mower, especially in the longer term, but if you feel it will become more convenient to use, that may be more important to you than maintaining it as a collector's piece.
