Welcome to TRS
No, the timing belt replacement can't cause the no starter motor activation, no cranking
I assume all the lights on the dash come on with Key On, like they should?
Starter motor activation is a separate system
Key turned to START(12volt out)---(fuse 24)-------neutral switch--------------------starter relay--------starter motor
No computer or anything else involved
On a manual trans the Neutral switch is on the clutch pedal, clutch pedal needs to be down all the way to activate starter
You should hear the Starter Relay on the inner fender "click" if the 12volts is making that far
If no "click" then check Fuse #24 in cab fuse box
On the Starter relay the smaller post with "S" next to it is to activate the relay
MAKE SURE TRANSMISSION IS
IN NEUTRAL
Use a jumper wire from Battery Positive and touch it to this smaller post
Relay should "click" and Starter should activate, even IN GEAR so BE CAREFUL
That will test if relay and starter motor are both working
If you get the "click" but no starter motor then starter is bad or relay was miswired