Hello everyone. I have recently started biking again for the first time in years and I have some questions. I had an older bike that was basically falling apart and long rides were hell. It wasn't worth repairing and someone gave me a 2001 Gary Fisher Wahoo to use while i get some cash together for a better bike. The issue is that I am 6' and the bike is a 13". I don't have any real issues riding it except for the steering being twitchy and being slightly top heavy. However I am concerned about my leg position taking a toll on me during long rides.

Now I know I should be riding a 19" and I will purchase one in the next few months, but until then I was wondering if a longer stem would be worth it to help me improve my position. I also used to have a curved seat post that moved the seat back on BMX bikes and was wondering if it would be terrible to use this on a MTB. Im not doing any offroad riding right now, just paved paths and streets.

Additionally, if I came across a good 19" frame for the right price, should all the parts from this fit most other frames? If not what are the limitations?

Thanks in advance for your advice.