You can successfully use pistons intended for many Yamaha 175cc bikes singles of the same era in the TY175. Some include 1973 CT3, 1974 and 1975 DT175 and all TY175. Wiseco pistons for these bikes are fine also. All these pistons provide the same compression ratio. Don't worry about the shape or size or even complete absence of the holes in the back of the piston.
More information needed about the whiskering please. First what do you mean by whiskering? We need to clarify this because you seem to be saying that the whiskering is "destroying" the plugs. If one of my plugs whiskered I would just flick the whisker off and put it back in again for more use.
Spark plug whiskers can be composed of steel or cast iron or carbon.
If you are getting steel whiskers it could be that the engine work wasn't very sanitary or the port edges weren't chamfered or there could be something rubbing metal to metal where it should not. Check a whisker with a magnet. If it sticks it is steel or cast iron.
If the whiskers are carbon and the engine has just been rebuilt, then there is something pretty amazing going on in there because carbon whiskers are usually (1) old carbon being loosened up by a change in engine operation ie lots of heat in an engine used previously for lots of gentle slow speed work or (2) a change in fuel type or premix oil type which loosens old carbon. If your engine has just been rebuilt, it shouldn't have any carbon deposits anyway.
I suggest you use an NGK B7ES or B6ES sparkplug, high quality fuel, high quality premix oil at between 25 and 35 to 1 ratio and when you get another whisker, see if you can work out what it is made of.
I just thought of something else. I remember a friend once added graphite to his premix on the advice of a graphite salesman and he managed to ride for about 20 minutes before the first carbon whisker formed on the sparkplug of his otherwise perfectly tuned bike. He cleaned the fuel system out and then spent the next few weeks replacing sparkplugs as the graphite (carbon) worked its way out of the engine!!
If you want more help, please provide more information about the way you ride, the fuel you use, the ignition timing, the carburettor type and size and jetting and how long in time the bike runs between whiskers.
David Lahey
Australia