1989 Toyota Pickup loss of antifreeze and heater won't heat

I replaced the radiator as it was leaking. My driving was for only a short distance so I never knew the truck was overheating. On a longer trip of about 10 miles, I noticed the temp guage rising quickly. I pulled over and let the engine cool for awhile. I then checked to see if there was any antifreeze. There was not, so I checked the oil stick and found it over filled with with a milky substance. This I assumed was antifreeze inside the engine. One mechanic said it could be a head gasket, or possibly a cracked block or even worse, but one couldn't tell without taking the head off. Another mechanic said he has seen this a few times with an r22 engine with 100,000 to 150,000 miles. His best guess is that the timing gear has probably worn through the plastic watershield(????) which is allowing antifreese into the engine. The engine is not running rough and is not making any noise. What do you think?? Any suggestions??