Engine sound is played when there are still enemies on the level if I recall correctly. Set read breakpoint to enemies counter, there are 2 of them.
Battle city doesn't have velocity values. It changes X/Y objects coordinates on some frames. Read frame counter address.
There are 2 addresses for powerups (I guess), one for tank graphics and one for actual tank power. Set correct values here when level loads and when a player dies. Also when died after grenade.
In original game there are fixed enemy numbers that spawn with bonus skin, which can be found by reading one of the enemies counters before they respawn. I think the same is for the hack, but more bonuses if there are 40 enemies.
Find battle city text in hex and set a read. You will have to write your oun title somewhere else if there is not enough space, and change some code here. There also should be a mechanic for drawing a next line, it's either a specific byte in text itself between words or a tiles counter for a string inside that code.