I got it working!
things I ran into:
ttgo-lora32-v1-868 board type did not pull the right libraries, so I added those. Also pin definitions were not entirely correct, so I hacked those into the header files 
the board seems to not have a lot of memory, so flash size was too big. Therefore I had to remove all rtl_433 protocols, except the bresser_5in1. now I am at 97.5% of the flash size.
So I did not do the changes in the intended way, but it works now 
thanks for your help to get me started!
here all my changes, in case somebody else wants to do it too. (or wants to do it the right way )
in platformio.ini I uncommented
default_envs = ttgo-lora32-v1-868
and in environments.ini I added (+) and removed (-):
lib_deps =
${com-esp32.lib_deps}
${libraries.lora}
+ ${libraries.rtl_433_ESP}
build_flags =
${com-esp32.build_flags}
- '-DZgatewayLORA="LORA"'
- '-DLORA_BAND=868E6'
- '-DGateway_Name="OMG_ESP32_LORA"'
+ ; *** OpenMQTTGateway Modules ***
+ '-DTTGO_LoRa32_v1=1'
+ '-DZgatewayRTL_433="rtl_433"'
+ '-DZradioSX127x="SX127x"'
+ '-DGateway_Name="OMG_ESP32_868_3"'
+ ; *** radio settings ***
+ '-DOOK_MODULATION=false'
+ '-DCC1101_FREQUENCY=868.30'
+ '-DSX1276_FREQUENCY=868.30'
+ '-DMY_DEVICES=true' ; subset of devices
in main.ino I added:
#define MY_DEVICES 1
#ifndef TTGO_LoRa32_v1
#define TTGO_LoRa32_v1 1
#endif
in rtl_sdr.h I changed line 137 :
#if defined(ARDUINO_TTGO_LoRa32_v21new) // LILYGO® Disaster-Radio LoRa V2.1_1.6.1
to
#if defined(ARDUINO_TTGO_LoRa32_v21new) || defined(TTGO_LoRa32_v1)// LILYGO® Disaster-Radio LoRa V2.1_1.6.1
in rtl_devices.h I modified the lines 254-262 to:
/**
* Subset of devices that I have access to and have tested with
*/
# define DEVICES \
DECL(bresser_5in1) \
/* Add new personal decoders here. */
# define NUMOF_OOK_DEVICES 0
# define NUMOF_FSK_DEVICES 1
#endif
and in signalDecoder.cpp I modified the section to include the bresser_5in1
#ifndef MY_DEVICES
// This is a generated fragment from tools/update_rtl_433_devices.sh
...
// end of fragement
#else
//memcpy(&cfg->devices[0], &lacrosse_tx141x, sizeof(r_device));
memcpy(&cfg->devices[0], &bresser_5in1, sizeof(r_device));**
#endif
in “C:\.platformio\packages\framework-arduinoespressif32\variants\ttgo-lora32-v1\pins_arduino.h” in
I added
#define LORA_IRQ 26 // GPIO26 - SX1276 IRQ (interrupt request)
#define LORA_D1 33 // GPIO33 - SX1276 IO1 (for LMIC Arduino library)
#define LORA_D2 32 // GPIO32 - SX1276 IO2
