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I have a Yamaha Special RX50K. It is a full sized bike, mid-sized frame street and trail motorcycle with a 50cc motor and 5 speed transmission. I was wondering if it was possible to fit a 125cc motor and new transmission into the same engine compartment. I am very new to motorcycles. I received this one from my grandfather not to long ago. It has been sitting in his shop for quite some time. I have the owners manual and tool pouch that came with it. It has 770 original miles on it. It is in very good condition and needs a little carb adjusting done to it which is not a problem. I hope to have some pictures up of it tomorrow. If anyone could give me any advice about fitting a bigger motor into this frame, it would be very helpful. By the way, I believe this motorcycle was made between the years of 1982 and 1984. Thanx.

You have an original bike with all the accessories.

The smartest thing you can do is sell the bike and use that money to purchase a 125cc bike. Why? Consider:

The bike is worth more than an average, beat up rx50

If you modify the bike, it will no longer be original, and will lose value.

You will spend money to modify the bike, but the bike will be worth less than its original value.

Do you see where I'm going? Save yourself the trouble - just buy the 125cc bike and sell the RX if it's not what you want.