Hey - how did I miss this one?
Absolutely first on the list - the Suzuki T/GT500 cannot be run on premix!!! Use the oil pump! You will need to drill the crankcase for oil flow if you want to go the premix route. Otherwise you will starve the crankshaft bearings.
The T500 and GT500 engines are practically the same. The GT has the PEI electronic ignition which required a different crank snout and different milling of the crankcases to fit the unit. Porting is milder than the early T500s but they did that around 1973 to distance the T500 from their new mid-sized triple the GT550. The GT500 also has a disc brake vs a drum on the T models. The tank is a GT750 unit and the seat pan and upholstery were changed to suit. This means the tank mount and seat mounts are different on the GT frame vs the T500 frame. Otherwise the frames and wheelbases are identical.
The GT500 can be run on the track without a battery as the PEI is a generating unit. You will need to disconnect the bulbs as the battery acts as a damper for them and you will burn out all of them with the battery out while running. The silver PEI controller boxes can burn out but the circuits have been mapped and they are not difficult to build. There's a good trouble shooting guide in the factory manual which you can download from here:
You should start with doing a pressure test on the engine to determine the shape of your crank seals. This will tell you if you need to split the engine open or not to have the crank rebuilt. You can build your own test set-up for about $25 or so. Let me know if you need details on this.
Seals are available. Here in the US three of the four seals are available along with the four o-rings needed. The right side crank seal has to be ordered from the UK. Not sure what your parts situation is like in Israel. Plus you will need someone that knows how to rebuild a two stroke crank.
Crank bearings and connecting rods have long been NLA from Suzuki. If you need the crank rebuilt and have to replace these items, they are available from Pete O'Dell in the UK:
TR500 Suzuki Specialist GT500, T500 Suzuki
Stainless chambers - really slip-on chambers like the old Allspeeds are available from Titan Performance in the UK. These seem to be good for street duty but you would want something that tucks in tighter like Swarbricks if you start racing your GT500:
www.classic2strokesuzuki.com - About us
Swarbrick Racing - Specialists in Competition Exhaust Systems - Lancashire - UK
The Swarbricks are in kit form and I do not know if he could supply you a stainless kit. They are excellent pipes when assembled by someone that knows what they are doing.
The T500/GT500 has a wide ratio gearbox which is not the best for racing duty. Nova Transmissions in the UK has both close ration gearbox kits as well as straight cut primary gears for the 500. It's all nicely made but expensive:
Nova Racing Motorcycle Transmissions/Gearboxes - Home
As for porting specifications, Muzza has his "old reliable" specifications that work well for someone starting out. In fact he has a full race prep section that may be of use to you:
Home of the Classic Suzuki Two Strokes Compendium T500 GT750 TR500 TR750
Suzuki 500 race preparation
And by the way, that's my GT500 race bike in my avatar.