The Stewart S-51D was developed by Jim and Peggy Stewart over a period of around 20 years. It started out as a wooden aircraft powered by a small block Chevy V-8 engine. Eventually Jim started over with an all aluminum design, and for a while he sold plans sets. As far as I know only one or possibly two builders actually completed and flew a plans-built aircraft, and the first was in the metal fabrication business. Jim further refined the design and changed from a small block Chevy V-8 to a big block, and in the early 90's completed the plans for the current S51 kits. At the time it was the most accurate replica of the P51D available, and represented a quantum jump in complexity and performance over the slightly smaller SAL and FEW kits. In my opinion it's still the most accurate replica, but after a couple of years it was eclipsed in performance by the Thunder Mustang. The carbon fiber Thunder Mustang was slightly larger and incorporated modifications directed at the air-racing market such as clipped wings.
The S51 made quite a splash at Oshkosh during this era. It was a truly unique aircraft, and for several years was featured in the aerobatic show. Elliot Cross did a great job of showing off its capabilities for warbird aerobatics. The combination of a unique product, great publicity, and an initial kit price which turned out to be far below the cost of production resulted in reasonable rate of kit sales. However, like most companies in the high performance end of the kit aircraft business, Stewart 51 eventually got into financial difficulty. There were many contributing factors, among them initially under pricing the kits, delivery problems with suppliers and the ubiquitous one of undercapitalization. The company soon acquired a deserved reputation as being unreliable in terms of delivery of kit components (for which the customers had already paid).
Eventually Jim Stewart "sold" the assets of Stewart 51, Inc. to Precision Aero Engineering of Camarillo, CA. PAE was founded to develop a V-8 engine suitable for the S51 and other similar projects. "Sold" is in quotes because PAE paid little or no cash for the S51 prototype, rights to the design, parts inventory, etc. In consideration for this they agreed to assume Jim Stewart's obligations to one of his principal suppliers and to the builders owed parts. Unfortunately but perhaps not surprisingly, PAE never made good on either of these obligations. Eventually they were evicted from their facility in Camarillo and most of their physical assets were sold at auction to satisfy various outstanding obligations, including a judgment for refund of some kit deposits.
In July 2007 a group of builders recovered all the rights to the S51, the prototype and all of the parts and tooling in the possession of PAE. Recovery was made by settlement of a breach of contract lawsuit. For the current status of the Stewart 51, visit the Stewart 51 Partner, LLC website.
Although you presently cannot buy a new S51 kit, from time to time you may be able to acquire an unfinished project or even a completed one. Many of the unfinished projects are missing some parts. Depending on what parts are missing and your abilities in fabrication and adaptation, this could be a serious problem or merely an annoyance. On the plus side, there is an active builder’s group and Stewart 51 Partner is providing what assistance they can with parts and plans. HPAI, the Czech company that did all the sheet metal fabrication for Jim Stewart, still has the tooling for the aircraft and will produce sheet metal parts for any legitimate license holder as needed. Builder's group members have found them to be a very reliable supplier.
At present I would recommend that anyone without the skill and equipment to fabricate machined and welded parts approach an incomplete S51 project with considerable caution. However if you the have aforementioned capabilities and a serious case of the Mustang bug, but like many of us are on a Cessna budget, you may find that the present situation offers some opportunities.