I have a '95 GS300, and have the same issue, of only having 1 key.
Lexus Exeter say there is a transponder and want to charge silly money to programme any new key.
Although I have had another locksmith look at it, and test it with a transponder detector and he says there is no transponder.
Have also checked the American site, where they say the transponders weren't fitted until '96.
Have hunted high and low to find a replacement key with 1 button, to no avail.
Let me know if you find one.
Please note the keys were made in 2 different lengths. You will discover that causes a problem.