Legally Configuring an HK Style Rifle in California
Most folks are acquiring HK clones based on the H&K 91 / G3 pattern. However, there are a number of domestic builders of H&K clones in the US:
Red Rock Arms, formerly known as Bobcat Weapons is best known for taking Special Weapons receivers and rebuilding them to... well, bring them up to spec. As of 2006, they have stopped working with Special Weapons, so no more HK clones from them.
Cohaire Arms is shipping MP5 clones now. They are off-list and available in California, in 9mm and 40S&W.
JLD Enterprises is best known for their PTR rifles, built on perfectly cloned G3 receivers. According to Xenophobe of Calguns, "The JLD Enterprises PTR-91 is the exact same receiver as the FMP G3S. JLD purchased all the machines FMP Portugal owned when they went out of business. Basically they are unlicensed copies made on real HK machinery."
Special Weapons is best known for their MP5 clones, but not for quality and worksmanship. If it's all you can get, though, it's all you can get.
Vector Arms produced a number of HK clones, including the very tempting V-51 in 308 - MP5 form factor, 308 caliber. Unfortunately, with parts kits drying up they stopped advertising them at some point prior to 9/5/2012.
Lastly, H&K itself has many rifles which were never listed! Namely, the HK41, HK911, SR-9 (and variants) are all based on the G3 rifle, and the HK43 (precursor to the HK93). None of these rifles are listed by name, and as long as the rifle is otherwise in compliance with California law, it is perfectly lawful to purchase these genuine H&K rifles.
Sources for MP5 Receivers
Special Weapons is no longer producing them, but used to produce MP5 receivers.
Rorschach, of Calguns, has a readily home-smithable technique to fix a PTR-91 / G3 magazine.
For those who prefer a "bullet button" approach, Radd of Calguns has developed a polymer sleeve system which works quite well.
There are also a lot of folks running around using 5/16" e-clips as a fixed magazine solution. These clips go in place right under the magazine release button. I personally do not feel safe with this fixed-magazine solution, and have discontinued its use. Presently, I'm leaving my rifle stripped down until I get around to fabricating a Rorschach style magazine device.
Modifying a pistol grip:http://www.robertrtg.com sells a used $15 trigger housing that you could cut the protruding pistol grip off of, transfer your trigger components over to the new cut-down housing, and you have a no-evil-features gripless rifle. This technique is meant for the PTR-91, but would also apply to MP5 clones, etc. Be aware that this trigger housing is originally for a full auto rifle, and as such must be trimmed in a few places to be installed on a semi-auto rifle. Also, be aware that it comes with 'SEF' markings - the equivalent of Safe/Semi/Auto. Depending on how cautious you are, you may be well advised to blacken out the 'F' marking with paint or a marker, so that law enforcement doesn't see it and just assume you have a fully automatic rifle.
Installing a kydex wrap:http://solartactical.com/ sells a variety of Kydex grip wraps, and they make one fitted for the various HK semi-autos. However, if you try to use it on a CA-94, you may find that it's far too loose for comfort (slides up/down a half inch or so). This article addresses what to do if your kydex sleeve is loose and you'd like to snug it up.