First, couple of pieces of background information for you. The PTR-91 is based upon the civilian version of the German G3. The G3 for Germany was made by the company Heckler & Koch. Hecker & Koch (often just HK) released a civilian version called the HK-91. The standard rear site is has four positions intended for 100 meters ~ 400 meters. The adjustment tool for elevation is not cheap and you'll need to google for it. Some sites, may have the tool. It is not simple tool to make, and some people have used a skinny-nose pair of pliers, but I find this less than ideal. The actual HK tool or a US copy is your best bet.
Check google again, but you may be able to find HK 91 manual or better yet the maintenance manual for the G3 and that will answer most of your questions.
The PTR-91 and the HK 91 are semi-auto only versions of the G3, much like the AR-15 is a semi-auto version of the M16.
As for mount there are basically two mount type choices. The Arms mount is claw mount for the G3 - Type rifles (G3, HK91, etc) unfortunately these are not cheap, between $120 ~ $250, the other is allen wrench screw based M1913 rail mount, lot cheaper, but interferes with mechanical sight system, depending on source in $25 ~ $100 range. Then any good scope should do you.
Since the PTR-91 is a .308 win (308 nato) round, and good scope for 308 rifle with weaver rings should get you setup.
There is a German scope and mount designed by the German army for the G3 that includes a claw-type mount and 4x power these run $400 ~ $600 depending on condition and source. But any good hunting scope and rings with the right mount will meet your needs.
There are number of choices here and lot depends upon how much you want to send and whether it is made by HK or is a clone. Your rifle is referred to as clone, as it was not manufactured by HK.
Don't let that bother you, as the PTR-91 is made with Portuguese tooling. The Portuguese Army once used the G3 as the standard military rifle, after changed rifles types, the Portuguese government which acquired a manufacturing license and tooling from HK, sold the tooling and parts to JLD, which became the PTR-91 with the addition of some US parts.
IMHO the PTR-91 is a quality rifle. But you need to spend sometime becoming familiar with the G3/HK91 platform and available options for it.
Buy and hold is my method of gun ownership!