insomniac jack

hi, 2 years ago, I bought a 1975 Dt125, which I found buried at a mates furniture workshop. it hadn't been on the road for 20 years, but was quite tidy considering. I got it going and rode it for the summer, and it went ok, but vibrated a lot over 50mph. last year I was too busy to do anything with it, but I am looking to get it tidied up for this summer. I have a few questions that I'm hoping someone may be able to help me with. Firstly, to enable me to buy the correct bits, I wonder if anyone could clarify what model this is? the engine number is prefixed with 1F9-, does this make it an "F" model? that would make sense, but from what ive read, its a bit early for that? secondly, having been used to bigger bikes, ranging from RD400, 350LC, XT250 to XS650, I am finding it a bit lacking when it comes to steep hills, reminds me of my FS1E! the gears seem to be long winded and I'm either revving the nuts off it, or its doing 2000 rpm. so I was wondering about sprocket sizes, or possibly a 175 conversion? is the 175 conversion a straight swap? I would assume the head would need to be changed as well? what about the carb? are they the same as the 125? any help would be appreciated!