2 things I'd try that are free.
1. Put the machine in switch test and smack it around like you're playing aggressively. Watch for switch hits.
2. If that still gives a tilt, start a game but pull the switch row connector after the ball is ejected. Play the ball best you can. If you get a tilt at this point you know it's the board.

This assumes no NC switches but I don't think there are any on this game.

If you're not sure which is the switch rows connector you can pull both. It would be rare that you had a voltage leak on a switch row but it can cause issues (accidentally soldered a switch with the machine on and the machine freaked out (don't do this!)).

Looking at the schematic here: http://pinball.flippers.info/driver1.pdf IC17 P2 is connected to C49 (hard to test) and R85. Test R85 to make sure it's not shorted.

If it is fine and IC11(pia) P10 is dancing correctly then IC17 should be replaced.

You can diode test P1&2 on IC17 against the values on IC18 P1&2 to see if they match.

good job finding it.