Why did you buy it and what are going to use it for?
Reel to reel tape machines are among the most complex pieces of audio equipment around, both from a mechanical and electrical point of view. Compounding the problem is that a lot of machines in the late seventies and pretty much all during the eighties had custom microprocessor and other IC chips to control the transport. These are long NLA and so the restorer is looking for salvage parts if they work.
Now I can not find a power switch for your machine. The belt is available. There is a lot of dismantling to change it. Yours is a three motor machine, but since it reverses there are two capstans and hence the drive belt.
Now the cracking is a very bad omen indeed. So this machine is quite likely not repairable and is a parts machine.
Now restoring a tape machine is a lot of work, and very expensive. It requires a lot of knowledge, equipment and patience. Most restorers will only work on machines with tubes or discrete transistors. These are earlier machines than yours.
Since yours is a non working machine it is worth $100 at most. A working machine like yours, but not restored is worth $350 to $500. Certified restored with warranty about $1000.
The cost of restoration by a skilled restorer is probably going to coast you $1000 and may be more, if you can find one. Most machines are restored by owner DIY enthusiasts. For a vintage enthusiast the value of the machine does not correlate to what they will spend on restoration!
Lastly you will really only be able to use that machine to play back prerecorded four track tapes. These never had the best quality as they were high speed copies. Making recordings will not likely be possible. No one makes new magnetic tape of the correct bias range for most machines of that era. ATR magnetics make very expensive tapes that require very high record bias. It is possible, but unlikely that your machine could be set up for that tape. So you would have to find a source of vintage tape if you want to record with it. Unfortunately every brand of tape had a different record bias spec. So to get good results you had to stick with one type of tape and a professional or knowledgeable owner with lots of expensive equipment could do the calibration.
If you do get it restored, set it up to use in the forward direction only. Do not use reverse. Turn the tape over. The tape has a different skew on the heads in forward and reverse. I have never worked on a reversing machine that you could get a perfect adjustment for both forward and reverse. So I set them up like a one direction machine.
In general the Far Eastern machines were very difficult to work on as they were thrown together. The Revox and the professional equivalent Studer machines are the best. Only Willi Studer could make tape heads with a response down to 20 Hz.
The American Ampex machines are also a good bet, but not so plentiful. The Revox A 700 below the Mk6 uses discrete transistors and these machines are a good bet for restoration. When I bought my non working Mk6 the oscillator chip that drives the direct drive capstan motor had failed. I had to patiently wait many months before finding a used replacement board.
If you really want to pursue this, and its not worth it, unless you want to get serious about reel to reel, then you should join the Tape Head forum.
This is certainly a fascinating hobby, but before getting involved make sure you have the desire and tenacity to pursue it. If you are going to count the cost it is not for you. If you do pursue it, you will end up restoring multiple machines because of differing tape speeds and track formats.
