Making the game harder

[MuriloMoreira]

Is there any way to make an RP1 in the game's stock solar system, something on the same difficulty?

[jimmymcgoochie]

Depends what you’re looking for- there are stock-ish implementations of KCT to add build times for your rockets, a variety of life support systems from the simple Snacks to the pretty comprehensive Kerbalism, a few part failures mods (Kerbalism does this too) and some mods to add crew training/health/stress/R&R. You have a choice of different tech trees, some of which put probe parts at the beginning for a sounding rocket/plane based start as you’ll find in RP-1.

There are also contract packs that replicate some real life missions in the stock Kerbol system which might interest you, different propellant types (e.g. cryogenic hydrogen and methane rockets) and so on, but the much smaller scale of the stock system makes it inherently easier to get around as even with the terrible fuel tank mass ratios, stock parts are overpowered for the stock system. 

I would suggest trying a combination of KCT, Kerbalism (with the full config for part failures, life support, radiation and science) and Probes before Crew and see how you get on.

[mateusviccari]

You can also get the kiwi tech three, along with its recommended mods and the ones @jimmymcgoochie suggested.