Use a piston stop -- install it in the #1 spark plug hole (passenger side front), turn the engine carefully until the piston hits the stop, mark the balancer where the timing pointer is, then spin the engine the other direction until it hits the piston stop again, and mark that point on the balancer. The spot exactly in the middle of those two points is TDC.
You can use a bolt as a piston stop. Just don't ram the piston too hard into it, you don't want any big divits in the piston which could cause hot spots. The hardest part will be rigging a way to turn the motor backwards -- if your balancer bolt isn't siezed up, it'll loosen when you try to turn the motor backwards. Also, you should remove all the spark plugs to make it easier to turn the motor (don't have to, but it'll take longer if you leave them in and it'll be a little harder to turn).