i think the gears in my lower unit are shot, she skips in reverse badly now and even sometimes in forward, also often cuts out when i engage in forward. anyway the drive is a 1978 pre alpha . i think its a MR but not sure. i have ordered a old pre alpha lower unit on ebay from a different motor set up and am hoping it will still fit mine. does the engine size matter for lower unit? i understand the gear ratio is changed in the upper unit?