In career mode, purchase the Volkswagen Jetta GLX VR6 from the Volkswagen dealership. Next, head into the Garage and the Buy Upgrades screen. Enter the "Engine and Power" section and purchase every engine upgrade available except for the Engine Swap (that includes each individual turbo upgrade, each intercooler, etc.). When you've finally purchased every upgrade, you can then purchase the second engine that's available in the Engine Swap for 2,500 Cr.

After purchasing the new engine, re-install the stock engine. You can then sell the second engine that you purchased for 44,092 Cr (remember, you only paid 2,500 Cr for it) for a profit. You can then buy the engine again (for 2,500 Cr), install the stock engine again, and sell the new engine again for 44,092 Cr. You can repeat this trick to build up as much money as you want.