Hi Ray. It's not crazy to start learning guitar at 70. I'm 68 and still learning and playing with all this recording. It's just fun stuff.
If you have "tape" outputs on your mini hifi, its a simple matter of taking a cable from that to the input channels of the PortaStudio. It's no more complicated than the old days when you plugged your cassette deck into a stereo receiver. Likewise, you can run the output of the Alesis to inputs on the Tascam. Then plug in your guitar or microphone and play along with the track.
You don't mention a microphone so I'm assuming you will be trying to run the Spark Amp directly to the Portastudio which might present a problem. The Spark doesn't have a line output, just headphone out. You would have to wire up a cable to do that.
I'm curious as to why you are using the tape PortaStudio. Is it something you've had for a while, or received from someone? I gave up on tape years ago, and never used cassette multitracks.
Your Spark Amp comes with USB output, which lets you feed directly to a computer. It also comes with recording software, so should be able to use your computer directly, meaning that you can drag an audio file to your program, select the guitar for another track and hit record. It's not terribly complicated once you take a little time. After it's set up, then doing it again is quick and easy.