By definition ... any 1911 that is not a true GI 1911A1 is a clone, including many Colt models. Colt produced the first commercial models in 1905, which were modified slightly from the original Browning design. In 1911, the US Govt adopted the pistol and designated it a "M1911". In 1924, the design was changed slightly and became a M1911A1. Since then, the patents ran out, which allowed any company to produce M1911A1s without violating patent laws. Colt continued to produce 1911s and called them "Commercial Models" or "Government Models" and the target models were called "Gold Cups". Later, Colt introduced the Commander, which were identical to a Govt Model except with a shorter slide/barrel. The Colt Officer's ACP shares the same basic design but has an even shorter slide/barrel and a shorter grip frame.
Today, the "Colt Series 70" has become the industry standard for 1911 type pistols. Nearly all aftermarket parts are said to be "Series 70 compatible". All clones have some variations that make them slightly different than the original M1911A1s. These include sights, triggers, extractors, firing pin interrupt devices, and slides plus many are available in stainless steel versus blued or Parkerized finishes.
Ruger SR 1911 clones have several departures from the original design to include the plunger tube, which is cast into the stainless steel frame rather than a separate part (a vast improvement). The sights are 3-dot medium profile and are set in a popular Novak dovetail. Some other Ruger enhancements are: beavertail grip safety, skeletonized hammer, extended thumb safety, extended trigger, extended magazine release, straight mainspring housing, beveled magazine well, enlarged ejection port, loaded chamber indicator, titanium firing pin, stronger tension firing pin spring, wood grips, an 8 round magazine plus a conventional 7 round, both with different magazine followers (also a vast improvement) and different serrations on the slide. With exception of the sights and plunger tube, all other parts are 100% compatible with Colt Series 70 parts, something that can't be said of many other clones.
Because the basic 1911 design is so robust, you can customize most any clone by installing aftermarket custom parts ... most being "drop-in". Ruger SR1911s already have most of the desirable enhancements. Some things you can do to further enhance a Ruger is to install a National Match grade barrel and bushing, install fully adjustable Novak type sights, extended slide lock, arched mainspring housing, or different grips. With some extra machining, you can install a fully ramped barrel. By installing a kit, you can convert a SR1911 to 9mm, 38 Super, 40 S&W, or 22 LR.
For my own SR1911, I swapped the straight mainspring housing for an arched housing that fits my hand much better. I also bought a matching Wilson Combat extended slide lock and thumb safety. My SR also works perfect with my Kimber 22 LR conversion kit.