Märklin has always had a small, trimmed-down version of their decoders, primarily to fit into very small locs, e.g., the 36xxx Köf. Until recently, this has been the 60902-based "mini C90x", which are load-regulated, but have no f1-f3 function outputs.
Unfortunately, these decoders have been increasingly used in larger locs, apparently to save some money. E.g., in the 37681 E18 and the 37401 E140 Cargo.

Since 2004, this mini/cheapy C90x appears to have been replaced with a decoder based on the ESU Lokpilot.

However, Märklin has further trimmed down this decoder (hence to be named Lokpilot Lite™) as compared to the Lokpilot Classic:
- 80 rather than 128 addresses
- No DCC mode – Motorola only
- No F1, F2, F3 in most decoders (F4 for acceleration/braking delay switch-off only, no shunting speed reduction)
Hard to understand why they went through the efforts to trim down an excellent €25 decoder!

Since some of the versions are PIC-programmed, tuning it by adding transistors for unwired F-outputs, as could be done with the 60902, is probably impossible.

The Lokpilot Lite -like decoders have come in at least 3 flavors:
The circuit board of the first version (M* part# 230063) is that of the ESU Lokpilot Classic v2.0 (2003-) and even carries the ESU mark.
The Lokpilot Lite PIC, based on the 16F676 PIC chip, comes in a very small version, but size apparently was not the purpose of its development, since it first appeared in the large SBB Ae3/6.
Also, a similar PIC-based version comes at a larger size, possibly to be able to fit into the existing decoder brackets.
Finally, there is a decoder that probably may be named Loksound Lite (likewise, 80 addresses and MotII only).

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Let's try to make a list of LP Lite-equiped locs:
(updated 28-11-04)

Lokpilot Lite / part# / available functions
37316 BR111 yellow/blue (part# 230063) F3/F4
37537 BR120 yellow/red (part# 230063) F1/F2/F4
37538 BR120 yellow/red (part# 230063) F1/F2/F4

Lokpilot Lite PIC – small version / part# / available functions
37510 SBB 3/6 brown (part# 611572) F4

Lokpilot Lite PIC – large version / part# / available functions
29750 BR212 Feuerwehr (series 2)* (part# 611574) F4

* since October 2004; 29750 series 1 (September) had a 60902 decoder

Loksound Lite
37540 BR55 (part# 610422) F1 through F8*

* To access with 6021, F1-4 use address 55; F5-8 use address 56

Update.
Things become more complicated every day.

Seems that M* have introduced yet another type of decoders:
based on the new Esu/M* mfx decoder, BUT... no mfx!

37770 SVT 137 is the first example.
Mfx circuit board layout, but no mfx, just Motorola II, 80 addresses only, no sound on board (separate old M* horn sound board). Circuit and components look exactly similar to the full mfx decoder. Would this mean that the mfx protocol could be reprogrammed into the chip?
How to call this one? NoMfx? sfx? mfx sans mfx? M* f 'x my mfx?

Mfx sans mfx / part# / available functions
37770 SVT 137 (part# 610423) F1(interior)/F3(horn*)/F4

* separate 1-channel sound board

Lokpilot Lite / part# / available functions
37316 BR111 yellow/blue (part# 230063) F3/F4
37537 BR120 yellow/red (part# 230063) F1/F2/F4
37538 BR120 yellow/red (part# 230063) F1/F2/F4

Lokpilot Lite PIC – small version / part# / available functions
37510 SBB 3/6 brown (part# 611572) F4

Lokpilot Lite PIC – large version / part# / available functions
29750 BR212 Feuerwehr (series 2)* (part# 611574) F4

* since October 2004; 29750 series 1 (September) had a 60902 decoder

Loksound Lite
37540 BR55 (part# 610422) F1 through F8*

* To access with 6021, F1-4 use address 55; F5-8 use address 56

ESU has released Loksound MFX as well, that supports Maärklin/Motorola II and Märklin System, but not NMRA-DCC. This seems to be a good choice if you want to upgrade existing locos and include sound.

The new Digital upgrade kits include an mfx-decoder, but what's the use with feedback if you still have to program the decoder manually? The reason for buying a set from Märklin is to get the 5-pole rotor and permanent magnet without having to order them as spare parts. But a LokPilot is half the price of the new fx-decoder from Märklin...