Re: 1988 85 force
If your boat has the same hull as the bowriders and cuddies, the limit is 85 HP. But the bowriders and cuddies have 16' 10" length, so if yours is truly 17' 6", it may take more HP. But I doubt it would be as high as 150; maybe 125. If you don't have the capacity placard, try posting your question in the Bayliner section. Someone there may have the same boat. Bayliner may be able to tell you the info, but won't be able to supply a replacement placard for a boat that old (I tried).
EDIT: From the title of your post, I assumed your boat is an '88. Is that correct?