Nobody makes the exact spare parts for these anymore.
It is the task of finding suitable alternatives.
There are sellers on the 'bay' selling belt kits for specific models, possibly at a higher price. For Antenna, try to salvage from any old radio that you may across.
Otherwise you can buy assorted belts (multiple sizes) online such as from aliexpress and find the best fitting belt from the lot. It is a tedious task, you are warned!
In any case, open the unit and have a survey first to identify what are required.
First is to make the electronics working, i.e. radio and Aux-in.
If the unit is stock and not messed with, the first and only resource you may need is contact cleaner.
I strongly recommend to go for Deoxit brand (cheapest will be DeoxIT® Fader, #F100L-L2C (Squeeze Tube)) or go for a F Series spray. This contact cleaner is with added lubricant, which will restore the electrical contact and retain the smooth feel for some time. (Ordinary Electrical contact cleaner will clean the contact initially, but will become dried up and make it worse later.)
Apply it at the contact points of all rotary/sliding controls and switches. There are many of these on a boombox including some on the PCB, including a long switch called the REC switch. Spray deoxit and activate the controls a hundred times to restore the contacts.
If successful, you can start enjoying the Aiwa from now on.
Keep restoration of cassette deck as the last priority, it is not easy. I think, this model has soldered wires all around that you need to remove the wires first before even you can reach the cassette mechanism. Opening cassette mechanism to reach the belts are also often 'suicidal'. No service manual of any boombox tell you how to replace the belts!
Take unlimited photographs (zoomed in well) while you open the box and access anything. It will help you to put it back as it was, and will be a life saver if anything gets cut or broken.
