The car does not have ABS, air-bag, only some have head rest and seatbelt. So, is there a way to workaround these limitations since they make the car not very safe?
The easiest thing to do is not buy the car in the first place if you are worried about these items.
There are a few aftermarket options for ABS, but most of those do not work very well. Since the vehicle was not equipped with them (with any type of option), retro-fitting them would be very difficult. Most ABS systems today work in conjunction with the PCM to cut engine power and such.
The airbag is about the same, but I don't believe there are any aftermarket solutions for this. If the car didn't come with it in some way/shape/form, it is going to be very difficult to get something into your vehicle. I suspect it would be possible, but you'd have to fit the entire airbag system, to include sensors, computer, and most importantly, steering wheel.
The easiest part of this would be the seats and seat belts ... just go aftermarket with some good Recaro's or other such seats. If money is a concern, you can always pick up a set of high back buckets from a junk yard or from someone with a wrecked vehicle and fit them to your new vehicle. This may take a bit of work, but it is more than doable.
Seat belts are about the same. The only difference is, if the vehicle didn't come with them, you'll probably need to figure out a way to securely bolt them into the vehicle. My suggestion would be to get some seat belts which would work in your situation from a donor car (may take some research), figure out the mounting points, then weld in a plate to the body (at each point) which will act as the base for the belt, drill a hole through the plate, then weld a nut on the back side of the plate which you'll bolt the seat belt to. It would just take some work to make it happen. Another option would be to mount a roll bar within the car and mount a 4- or 5-point harness off of it. Either way will get you what you are seeking.
On the mechanical side, if there is the need to get spare parts, are they easy to find? And what about upgrades? Not engine swap level, but maybe the intake, filters, suspension...
These questions are really relative to where you live in the world and which parts you are talking about. Most wear items (brakes, windshield wipers, etc) are going to be fairly easy to get if you live here in the States. An engine may not be as easy to get. The easiest thing to do here would be to swap in a newer engine and transmission, which would also give you a performance aspect. These cars are rear wheel drive, so finding a small 4-cylinder engine which would work in the vehicle, may be a little tough, considering most engines/trannys are made for front wheel drive setup.
As far as upgrades, there are plenty of bolt-ons available, such as cold air intakes (CAI) or modifying the exhaust. You need to face the fact, though, that most of these cars were never meant to be speed demons. They were meant to be budget grocery getters. The only modification which you could do to truly increase performance without changing an engine is to add a turbocharger. While adding this is in no way simple, it would add some go to your little buggy. If you do go this route, you'll want to think about supporting modifications, such as a better carburetor and exhaust to handle the need of getting air in and exhaust out.