jim wrote:
1. I have no access to the tub taps from the rear. Outside wall
2. The previous owner never purchased extra tile for spares when they
tiled the entire bathroom - can't match them up.
3. The valves are Waltec. The valve seats are fine. The installer
placed the assembly too far into the tile. The only way to turn the
water off is to keep the handle screw loose as the tile prevents the
tap from fully closing when the handle is tightly seated.
4. This causes the handle to strip out. Always.

Is there a way to replace or modify without the a completed tile job?

I had free help give me the same problem. Like others said, either use
washers/spacers and a longer screw, or cut/file,grind down the back side
of the knob where it it hitting.