I had a similar issue as you with my 1996 35 triple. Engine basically stayed in slow mode. Ordered a New Power Pack and the issue remained. The ultimate cause was in the wiring harness. Ended up cutting the tan wire for the low oil / no oil warning at the power pack connector. There are two tan wires one is for the over temp and the other is for oil system. To see if it is the wiring harness you need to disconnect the connector with the tan wires if the motor runs with no issues there is your problem.
Thanks hidef! Wish I would've seen this about 24 hours ago... lol. But I appreciate the info. May ask the shop to look into that if the PowerPack does not resolve this. That's basically what I asked from the original concern about the oil tank being removed and if anything could go wrong or needed to be done to the wiring for the low oil sensor possibly putting the motor into SLOW. Great info though!