The following are low performance replicas. Although they do not begin to approach the performance of the P-51, they are relatively inexpensive, look reasonably good and are currently in production. Probably they are a lot of fun to fly. The Loehle 5151 is of wood and fabric construction. The T-51 is a more recent addition. It has a tube frame and aluminum skins. I think they are both in the 65-70% scale range.
The following kits are in the intermediate performance range at about 65% scale. The SAL Mustang has been around for a long time. When powered with a small block V-8 instead of the original Ranger engine, it has good performance. As I recall it is constructed from a variety of materials, including aluminum, wood, fiberglass and fabric. It is reported to be based on the 75% scale Jurca plans. Plans and materials (including some fabricated components) are available from Falconar Avia in Edmondton. Some SAL builders seem to hang out on a Yahoo group loosely affiliated with the Replica Figuters Association. There is also a SAL e-mail list hosted by Yahoo/groups.
The FEW P-51 has also been around for several years, originally in a single seat version and later in the TF-51 two seat version. These are fiberglass aircraft powered by small block or LS series V-8s. The design has changed hands since Task developed it and is now manufactured by Legendary Aircraft. I'm told that production has resumed and kits are available. For the latest word you can contact the new owner, Ed Hullinger at firstname.lastname@example.org or see the webpage below.
The Jurca MJ-7 Mustang must be built from plans. It is tube and fabric with a wooden wing. I think most builders use smaller V-8s, either a small block or a Ford. In English speaking countries, plans are availabe from Michael Heit; (509) 253-4758. You can also subscribe to his Yahoo group .
Builders contemplating an experimental aircraft utilizing a V-8 engine (or for that matter any non-certified engine) should keep in mind that in the current environment insurance may be somewaht difficult to find.
The Stewart and the Thunder Mustangs are in the high performance category. Both will replicate some aspects of the performance of the real P-51. The Thunder Mustang is a little larger, probably around 80% scale, and is made from carbon fiber. The design seems to have been optimized mainly for air racing (clipped wings for example), and it is significantly faster than the Stewart. The original engine used was a Facloner V-12. As I understand it considerable difficulty has been encountered with this engine, and you can find several "firewall back" projects advertised on the Thunder web page. Neither the Stewart or the Thunder kits are currently in production, and in both cases some of the projects out there are missing important parts.
Jurca plans (MJ-77) are also available for a full scale replica P-51, again constructed from wood, tube and fabric. The full scale Jurca projects are usually powered by Allison V-12s. These should surpass the performance of the real aircraft in some respects.