If you received the original engine documents from the first owner you will have a CH ops manual. It would normally have all the instructions on how to set timing and a copy of a timing protractor. If not you can download all of it from the CH website. It's easy to check the timing. BME was known for some excellent machine work. So good that you often could not see where a ring or other part was movable. Look to see if there's some set screws that when loosened would permit the part that the hall sensor is mounted on to move or rotate. I can't tell from the picture how your hall sensor is mounted. If you feel up to it a couple more pics could help.

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