If you don't want to mod your datasette,check out
1530USB Adapter also this
USB Tape Interface.
This mod requires some soldering skills.
You will be able to load games to your C64 using a PC,phone,tablet or MP3 player.
You can backup your original cassette tapes to your PC.
You can record games to blank cassette tapes.
These instructions for the datasette with the board code
HCT-4 94HB NP-090BIf you have a different board,post a picture of it.I will have a look if I have the same one.
LINE OUTLine out mod is for backing up/dumping cassette tapes to your PC.You can save it as wav with Audacity or convert it to tap with Audiotap.
You need to plug this into your PC's microphone in port.If you have a Line-in instead you might need to lower the resistance.
You will need a 100nF capacitor and a 22K resistor.If you open up an old radio,PC speakers or any audio equipment,you can salvage these parts.
LINE IN for LOADINGLine in mod is for loading games to your C64 from an audio source.
You can plug in to your PC's speaker out and use the Audiotap or WAV-PRG to load sotware to C64.
LINE IN for RECORDINGThis mod is for recording games to cassette tapes.Straight solder on job.No components needed.
Plug this into your PC's speaker output port.
If you want all above mods on your datasette,there will be 3X3.5mm audio jacks coming out of it.
If you have a newish laptop with mic and speaker combined;
You can use a 4 pole 3.5mm connector for Line-out and Line-in for loading.But you still need another connector for the Line-in for recording.
A simple SPDT switch can be installed on the datasette marked as LOAD and RECORD.I will try to find other solutions to avoid drilling the datasette.