demoilsf - if u are just looking at parking lot fun , and as u say "not at a level yet for a high performance rear exhaust engine, then i would recommend nothing but OS brand engines. They hold a tune better than anything out there, have great support from the LHS's, are VERY fast, and are reasonably priced. Take a look at the OS .12cv-rx or the OS .15cv-rx, both have a side exhaust and a pull start.
You mentiona 70mph top speed - let me enlighten you on something - people always think that their RC cars go way faster than they actually do. Most high perforance cars, such as the NTC3 and serpent impulse when equipped with a strong engine, such as a mugen or OS will rarely be able to reach 60mph. if you search the web, you will find that most good TC's will cruise around 50-55. It is possible to reach those higher speeds, but not without serious mods to suspension, an extreme engine and VERY tall gearing. What you gain the most with a stronger mill is usualy low end power, and better acceleration through the power band. You will be much happier with a better engine, but unless your previous mill is trash, u will not notice an extreme increase in top end. 10-15 mph would be phenominal. Good Luck, and check out those OS engines