10mm allen under the solenoid
I just managed to do this last weekend on my 230 - sorry for not noticing sooner. I used a 10mm allen wrench, cut the angle part off, and put in in a 10mm deep socket with a flex joint on the end of the socket. Battery box makes it real tight, but I managed without pulling it. The bolt is directly across the starter from the bottom one, and you have to wiggle the 10mm in (I did it with my hand on the deep socket until it seemed to seat, and then connected the flex socket and a long handled 3/2 flexhead driver).
When I put it in (after many tried), I "learned" to put the bolt and allen/socket combo in right next to the solenoid, and put a piece of masking tape on it to hold it while I installed the starter. PITB, but it works.