Life long computer nerd here. There is no magic bullet, but a little info can help understand the pieces.
A NMEA 2000 (or N2K) network is a wired cabling system that lets everything connected to it send and receive little text messages to all the other connected devices. Your Garmin MFD (which model?) knows how to listen for tachometer messages or engine temp messages and will display the values in the messages if it sees them.
Your fuel and temp senders work by creating some resistance depending on the fuel level or the engine temp. For example, fuel senders often show about 33 ohms for full and 240 ohms for empty. The gauges read that resistance and convert that resistance to dial movement. All analog and old school electronics. To see this info on the Garmin MFD, you need some way to convert electrical resistance to NMEA messages.
There are devices that can convert electrical resistance readings into NMEA 2000 messages. You don’t have to replace your senders or gauges. One such device is the
Actisense EMU-1. It costs around $450. It also requires the
Actisense NGT-1 (USB version) ($225?) to program the EMU-1 and a PC to do the programming. This is an expensive device and it is also a very generic conversion device. It can be connected to many kinds of engine senders.
The EMU-1 also has dedicated input connectors for connecting a tachometer.
Lastly you need to figure out how to configure the EMU-1. The EMU-1 has 6 gauge inputs, but they are generic inputs. To configure the EMU-1, you use the PC software that comes with the EMU-1 to say what engine sender is connected to each input. For example, you might connect gauge input #1 on the EMU-1 to the fuel sender. You use the PC software to set gauge input #1 to send NMEA 2000 fuel level messages, uand when the input reads 33 ohms broadcast tank full messages, and when it reads 240 ohms broadcast tank empty messages.
You‘ll also need to run NMEA cables between the Garmin and the EMU-1 and there are some details like cable termination that need to be followed.
If you are willing to pay for the Actisense devices, it might be time to talk with your computer nerd friends and see if they are willing to help out with the configuration. They might consider it an interesting challenge.
There are other engine monitoring devices available. The EMU-1 is just one example. But they all involve understanding how to connect your senders to the monitoring device and how to perform the configuration.