first of all, its not that hard to reseal the upper, it may take some creativity and or special tools, but its not that hard. Go ahead and pull teh upper off the boat, flip it upside down, use an extra long drill bit and drill out the broken bolt, if you screw it up, you wouldn't have wasted time resealing before hand. After you get the broken scew taken care of, pull the top cap, remove the big spanner nut for the drive shaft(here's where that creativity or special tool comes in), keep the spacers and shims in place( I use a bread tie to keep them together till I'm ready to put them back in). Once you have the upper stripped, knock all teh seals out, take notice of the position of the lip(take pics if you have to). The fun part is driving the new seals in place with out the proper drivers, you can experament with various pieces of tubing or other objects, worse comes to wrose, take it to a shop, cleaned up, with everything ready to go back together, and see what they will charge to reaseal and assemble it.
As far as the aftermarket drives, the uppers are essitially the same from MC1, MR1, and Alpha 1(gen 1), there is only cosmetic external changes on the uppers of these drives. What makes an Alpha, an Alpha( and 7 times stronger than the earlier drives) is all in the lower, so even if you were to upgrade to a high output V8, you would really only need to upgrade the lower. Remember, they used to offer 230 hp(around 200 by todays standards) 351 Fords with the MCI
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