No idea about '87, but Merc apparently used either Ford or GM 3.0's at times.
Pertronix doesn't seem to list anything specifically for Mercury 188 in their marine conversion kit section of the catalog: ( Pfd
p 18 ) http://www.pertronix.com/catalogs/pd...tronix2010.pdf
If yours by any chance has a Mallory YM or HP advance distributor, Mallory Performance lists 2 different 4 cyl marine conversion modules for 2 diff. cap styles , "high-stack and "mini-bowl": http://www.malloryperformance.com/Pr...minselection=4
(bottom two.) http://www.malloryperformance.com/pdf/561_562_563.pdf
Hopefully sjef can fix you up with one of the marine electronic replacement distributor kits with elect. distributor & matching coil & wires, which would likely be the best but pricier way to go on an engine with a 23 yr old distributor & advance.
FWIW- MichiganMotorz has the really good Delco Voyager EST Electronic Distributor kit #107A for a 3.O L engine (GM) for around $200.
I used the Delco Voyager EST kits on my old 5.7L engines and they were terrific. Wished I ( actually the yard) had put them on years ealier. I think they were the ony marine replacement electronic distributors that had electronic advance, not mechanical .